rcu: Permit limiting of force_quiescent_state() latency
RFC PATCH 1/1 rcu: Permit limiting of force_quiescent_state() latency
Hello, Dimitri and Mike,
This one is a first step in limiting RCU-induced latencies for systems that have not just NR_CPUS=4096, but also a lot of CPUs. I have tested this using my big-system-simulation setup, but of course testing on systems that really are big would be quite valuable.
Remaining issues include the latency of setting up grace periods and the latency of finalizing them in the case where another grace period is not immediately rquired.
Thanx, Paul
------------------------------------------------------------------------
Systems whose cache-miss penalties are large compared to the permitted per-CPU force_quiescent_state() latency can limit these latencies by defining CONFIG_ARCH_RCU_FQS_LIMIT to be the maximum number of CPUs that force_quiescent_state() is permitted to visit per invocation. If this limit is exceeded, the next CPU to scan is stored into a new ->fqs_next_cpu field in the rcu_state structure, and the existing ->jiffies_force_qs field is set so that the next CPU will re-invoke force_quiescent_state() rather than waiting a few jiffies.
kernel/rcutree.c +33 -9
@@ -1685,47 +1685,66 @@ void rcu_check_callbacks(int cpu, int user)* have not yet encountered a quiescent state, using the function specified. * Also initiate boosting for any threads blocked on the root rcu_node. * +* Returns 0 if the scan could not be completed, 1 otherwise. * The caller must have suppressed start of new grace periods. */ -static void force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) +static int force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) {unsigned long bit;int cpu; +int cpusvisited =0 ;unsigned long flags;unsigned long mask;struct rcu_node *rnp;rcu_for_each_leaf_node (rsp, rnp) { +if (rnp->grphi < rsp->fqs_next_cpu) +continue ; mask =0 ;raw_spin_lock_irqsave (&rnp->lock, flags);if (!rcu_gp_in_progress (rsp)) {raw_spin_unlock_irqrestore (&rnp->lock, flags); -return ; +return 1 ; }if (rnp->qsmask ==0 ) {rcu_initiate_boost (rnp, flags);/* releases rnp->lock */ continue ; } cpu = rnp->grplo; +if (cpu < rsp->fqs_next_cpu) + cpu = rsp->fqs_next_cpu; bit =1 ;for (; cpu <= rnp->grphi; cpu++, bit <<=1 ) { -if ((rnp->qsmask & bit) !=0 && -f (per_cpu_ptr (rsp->rda, cpu))) +if ((rnp->qsmask & bit) ==0 ) +continue ; +if (cpu >rcu_get_max_cpu ()) + goto done; +if (f (per_cpu_ptr (rsp->rda, cpu))) mask |= bit; +if (++cpusvisited >=FQS_MAX_CPU_VISIT ) { + rsp->fqs_next_cpu = cpu +1 ; +break ; + } }if (mask !=0 ) {/* rcu_report_qs_rnp() releases rnp->lock. */ rcu_report_qs_rnp (mask, rsp, rnp, flags); -continue ; +if (cpusvisited <FQS_MAX_CPU_VISIT ) +continue ; }raw_spin_unlock_irqrestore (&rnp->lock, flags); +if (cpusvisited >=FQS_MAX_CPU_VISIT ) +break ; } rnp =rcu_get_root (rsp);if (rnp->qsmask ==0 ) {raw_spin_lock_irqsave (&rnp->lock, flags);rcu_initiate_boost (rnp, flags);/* releases rnp->lock. */ } +done: + rsp->fqs_next_cpu =0 ; +return 1 ; }/*
@@ -1735,6 +1754,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))static void force_quiescent_state (struct rcu_state *rsp,int relaxed) {unsigned long flags; +int retval;struct rcu_node *rnp =rcu_get_root (rsp);trace_rcu_utilization ("Start fqs" );
@@ -1771,21 +1791,25 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)raw_spin_unlock (&rnp->lock);/* irqs remain disabled */ /* Record dyntick-idle state. */ -force_qs_rnp (rsp, dyntick_save_progress_counter); + retval =force_qs_rnp (rsp, dyntick_save_progress_counter);raw_spin_lock (&rnp->lock);/* irqs already disabled */ -if (rcu_gp_in_progress (rsp)) +if (rcu_gp_in_progress (rsp) && retval) { rsp->fqs_state =RCU_FORCE_QS ; + rsp->fqs_next_cpu =0 ; + }else if (rcu_gp_in_progress (rsp)) + rsp->jiffies_force_qs = jiffies -1 ;break ;case RCU_FORCE_QS :/* Check dyntick-idle state, send IPI to laggarts. */ raw_spin_unlock (&rnp->lock);/* irqs remain disabled */ -force_qs_rnp (rsp, rcu_implicit_dynticks_qs); + retval =force_qs_rnp (rsp, rcu_implicit_dynticks_qs);/* Leave state in case more forcing is required. */ -raw_spin_lock (&rnp->lock);/* irqs already disabled */ +if (rcu_gp_in_progress (rsp) && !retval) + rsp->jiffies_force_qs = jiffies -1 ;break ; } rsp->fqs_active =0 ;
kernel/rcutree.h +16 -0
@@ -348,6 +348,21 @@ do { \ }while (0 )/* +* Large latency-sensitive configurations can limit force_quiescent_state() +* latencies by defining an CONFIG_ARCH_RCU_FQS_LIMIT. This should be +* sized based on that architecture's cache-miss latency and the maximum +* desired force_quiescent_state latency. For example, if the cache-miss +* latency was 100 nanoseconds, and the maximum force_quiescent_state() +* latency contribution was 5 microseconds, then that architecture should +* define CONFIG_ARCH_RCU_FQS_LIMIT to be 50. + */ +#ifdef CONFIG_ARCH_RCU_FQS_LIMIT +#define FQS_MAX_CPU_VISIT CONFIG_ARCH_RCU_FQS_LIMIT +#else /* #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ +#define FQS_MAX_CPU_VISIT NR_CPUS +#endif /* #else #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ + +/* * RCU global state, including node hierarchy. This hierarchy is * represented in "heap" form in a dense array. The root (first level) * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
@@ -396,6 +411,7 @@ struct rcu_state {/* or NULL if no barrier. */ raw_spinlock_t fqslock;/* Only one task forcing */ /* quiescent states. */ +int fqs_next_cpu;/* Next CPU for fqs to scan. */ unsigned long jiffies_force_qs;/* Time at which to invoke */ /* force_quiescent_state(). */ unsigned long n_force_qs;/* Number of calls to */
Quoted 6 lines · Paul E. McKenney
Hello, Dimitri and Mike,
This one is a first step in limiting RCU-induced latencies for systems that have not just NR_CPUS=4096, but also a lot of CPUs. I have tested this using my big-system-simulation setup, but of course testing on systems that really are big would be quite valuable.
This didn't work for me.
On a 48 core DL785, with your GP initialization patch applied, and tracing with irqsoff tracer with box idling along, tracer reports > 200us latencies (tracer overhead alert) from force_qs_rnp() banging on 254 locks.. iow it didn't stop traversing, did a good imitation of a latency spike.
So I bent it up a little, see below. I haven't instrumented it yet, and might have busted it all to pieces, so RFC back.
kernel/rcutree.c void rcu_check_callbacks(int cpu, int user) · line 1690
Quoted 11 lines
kernel/rcutree.c +1
-0@@ -1685,47 +1685,66 @@ void rcu_check_callbacks(int cpu, int user)* have not yet encountered a quiescent state, using the function specified. * Also initiate boosting for any threads blocked on the root rcu_node. * +* Returns 0 if the scan could not be completed, 1 otherwise. * The caller must have suppressed start of new grace periods. */
But it fibs, always returns 1.
Quoted 11 lines
kernel/rcutree.c +2
-1-static void force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) +static int force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) {unsigned long bit;int cpu; +int cpusvisited =0 ;unsigned long flags;unsigned long mask;struct rcu_node *rnp;rcu_for_each_leaf_node (rsp, rnp) {
.....
kernel/rcutree.c
Quoted 7 lines
for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {kernel/rcutree.c +4
-2-if ((rnp->qsmask & bit) !=0 && -f (per_cpu_ptr (rsp->rda, cpu))) +if ((rnp->qsmask & bit) ==0 ) +continue ; +if (cpu >rcu_get_max_cpu ()) + goto done;
No unlock.
....
kernel/rcutree.c
Quoted 4 lines
kernel/rcutree.c +3
-0+done: + rsp->fqs_next_cpu =0 ; +return 1 ; }
rsp->fqs_next_cpu = 0 means we start over, no?
WRT FQS_MAX_CPU_VISIT, maybe a read_mostly tweakable instead? (Like we don't have enough tweakables lying about)
Anyway, this is what I did. Setting ARCH_RCU_FQS_LIMIT to 64 on a 48 core box didn't test sip vs gulp logic particularly well, but that doesn't matter ATM. The only test done was to boot it and see if it stopped banging on oodles of locks.
Ok, box has been running rcutorture for ten whole minutes now without deadlocking or exploding.. ship it, and go have part of a weekend.
<poof>
rcu: Permit limiting of force_quiescent_state() latency
Systems whose cache-miss penalties are large compared to the permitted per-CPU force_quiescent_state() latency can limit these latencies by defining CONFIG_ARCH_RCU_FQS_LIMIT to be the maximum number of CPUs that force_quiescent_state() is permitted to visit per invocation. If this limit is exceeded, the next CPU to scan is stored into a new ->fqs_next_cpu field in the rcu_state structure, and the existing ->jiffies_force_qs field is set so that the next CPU will re-invoke force_quiescent_state() rather than waiting a few jiffies.
<snip SOB>
--- arch/x86/Kconfig | 4 ++++ kernel/rcutree.c | 48 +++++++++++++++++++++++++++++++++++++++--------- kernel/rcutree.h | 16 ++++++++++++++++ 3 files changed, 59 insertions(+), 9 deletions(-)
arch/x86/Kconfig +4 -0
@@ -254,6 +254,10 @@ config ARCH_CPU_PROBE_RELEASE def_bool y depends on HOTPLUG_CPU && !XEN +config ARCH_RCU_FQS_LIMIT + int + default 64 + source "init/Kconfig" source "kernel/Kconfig.freezer"
kernel/rcutree.c +39 -9
113 lines not on this page · raw message
kernel/rcutree.h +16 -0
@@ -354,6 +354,21 @@ do { \ }while (0 )/* +* Large latency-sensitive configurations can limit force_quiescent_state() +* latencies by defining an CONFIG_ARCH_RCU_FQS_LIMIT. This should be +* sized based on that architecture's cache-miss latency and the maximum +* desired force_quiescent_state latency. For example, if the cache-miss +* latency was 100 nanoseconds, and the maximum force_quiescent_state() +* latency contribution was 5 microseconds, then that architecture should +* define CONFIG_ARCH_RCU_FQS_LIMIT to be 50. + */ +#ifdef CONFIG_ARCH_RCU_FQS_LIMIT +#define FQS_MAX_CPU_VISIT CONFIG_ARCH_RCU_FQS_LIMIT +#else /* #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ +#define FQS_MAX_CPU_VISIT NR_CPUS +#endif /* #else #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ + +/* * RCU global state, including node hierarchy. This hierarchy is * represented in "heap" form in a dense array. The root (first level) * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
@@ -391,6 +406,7 @@ struct rcu_state {/* starting new GP. */ raw_spinlock_t fqslock;/* Only one task forcing */ /* quiescent states. */ +int fqs_next_cpu;/* Next CPU for fqs to scan. */ unsigned long jiffies_force_qs;/* Time at which to invoke */ /* force_quiescent_state(). */ unsigned long n_force_qs;/* Number of calls to */
Quoted 1 line · Mike Galbraith
So I bent it up a little, see below.
s/bent/busted. FQS_MAX_CPU_VISIT < CPUs == deadlock. Poo.
-Mike
Quoted 24 lines · Mike Galbraith
kernel/rcutree.c +7
-1@@ -1316,47 +1316,72 @@ void rcu_check_callbacks(int cpu, int us* have not yet encountered a quiescent state, using the function specified. * Also initiate boosting for any threads blocked on the root rcu_node. * +* Returns 0 if the scan could not be completed, 1 otherwise. * The caller must have suppressed start of new grace periods. */ -static void force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) +static int force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) {unsigned long bit;int cpu; +int cpusvisited =0 , done =1 ;unsigned long flags;unsigned long mask;struct rcu_node *rnp;rcu_for_each_leaf_node (rsp, rnp) { +if (rnp->grphi < rsp->fqs_next_cpu) +continue ; +if (rnp->grplo >rcu_get_max_cpu ()) +continue ;
...
kernel/rcutree.c
Quoted 9 lines
if (mask != 0) {kernel/rcutree.c +2
-1/* rcu_report_qs_rnp() releases rnp->lock. */ rcu_report_qs_rnp (mask, rsp, rnp, flags); -continue ; + if (cpusvisited < FQS_MAX_CPU_VISIT) deadlock: if no continue.. +continue ; } raw_spin_unlock_irqrestore(&rnp->lock, flags); _thoroughly_ unlock it.
Boots/runs. Altered to not bail mid-group.
I tried a synchronized jitter test on 45 cores, patches made diddly spit difference. There's too much other noise. Also applied both patches to RT, and tested on 60 cores, which also made little if any difference, but then jitter is single digit (usecs) running this test with the RT kernel already. With NOPREEMPT kernel, jitter is low triple digit worst case with or without patches.
--- arch/x86/Kconfig | 4 ++++ kernel/rcutree.c | 49 ++++++++++++++++++++++++++++++++++++------------- kernel/rcutree.h | 16 ++++++++++++++++ 3 files changed, 56 insertions(+), 13 deletions(-)
arch/x86/Kconfig +4 -0
@@ -254,6 +254,10 @@ config ARCH_CPU_PROBE_RELEASE def_bool y depends on HOTPLUG_CPU && !XEN +config ARCH_RCU_FQS_LIMIT + int + default 16 + source "init/Kconfig" source "kernel/Kconfig.freezer"
kernel/rcutree.c +36 -13
110 lines not on this page · raw message
kernel/rcutree.h +16 -0
@@ -354,6 +354,21 @@ do { \ }while (0 )/* +* Large latency-sensitive configurations can limit force_quiescent_state() +* latencies by defining an CONFIG_ARCH_RCU_FQS_LIMIT. This should be +* sized based on that architecture's cache-miss latency and the maximum +* desired force_quiescent_state latency. For example, if the cache-miss +* latency was 100 nanoseconds, and the maximum force_quiescent_state() +* latency contribution was 5 microseconds, then that architecture should +* define CONFIG_ARCH_RCU_FQS_LIMIT to be 50. + */ +#ifdef CONFIG_ARCH_RCU_FQS_LIMIT +#define FQS_MAX_CPU_VISIT CONFIG_ARCH_RCU_FQS_LIMIT +#else /* #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ +#define FQS_MAX_CPU_VISIT NR_CPUS +#endif /* #else #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ + +/* * RCU global state, including node hierarchy. This hierarchy is * represented in "heap" form in a dense array. The root (first level) * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second
@@ -391,6 +406,7 @@ struct rcu_state {/* starting new GP. */ raw_spinlock_t fqslock;/* Only one task forcing */ /* quiescent states. */ +int fqs_next_cpu;/* Next CPU for fqs to scan. */ unsigned long jiffies_force_qs;/* Time at which to invoke */ /* force_quiescent_state(). */ unsigned long n_force_qs;/* Number of calls to */
Quoted 42 lines · Mike Galbraith
Quoted 24 lines · Mike Galbraith
kernel/rcutree.c +7
-1@@ -1316,47 +1316,72 @@ void rcu_check_callbacks(int cpu, int us* have not yet encountered a quiescent state, using the function specified. * Also initiate boosting for any threads blocked on the root rcu_node. * +* Returns 0 if the scan could not be completed, 1 otherwise. * The caller must have suppressed start of new grace periods. */ -static void force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) +static int force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) {unsigned long bit;int cpu; +int cpusvisited =0 , done =1 ;unsigned long flags;unsigned long mask;struct rcu_node *rnp;rcu_for_each_leaf_node (rsp, rnp) { +if (rnp->grphi < rsp->fqs_next_cpu) +continue ; +if (rnp->grplo >rcu_get_max_cpu ()) +continue ;...
kernel/rcutree.c void rcu_check_callbacks(int cpu, int us
Quoted 9 lines
if (mask != 0) {kernel/rcutree.c +2
-1/* rcu_report_qs_rnp() releases rnp->lock. */ rcu_report_qs_rnp (mask, rsp, rnp, flags); -continue ; + if (cpusvisited < FQS_MAX_CPU_VISIT) deadlock: if no continue.. +continue ; } raw_spin_unlock_irqrestore(&rnp->lock, flags); _thoroughly_ unlock it.Boots/runs. Altered to not bail mid-group.
Quoted 6 lines
I tried a synchronized jitter test on 45 cores, patches made diddly spit difference. There's too much other noise. Also applied both patches to RT, and tested on 60 cores, which also made little if any difference, but then jitter is single digit (usecs) running this test with the RT kernel already. With NOPREEMPT kernel, jitter is low triple digit worst case with or without patches.
Thank you very much for your efforts on this!!!
Given that you were seeing about 200-microsecond latency spikes on grace-period initialization, I would expect that you would need about 200 dyntick-idle CPUs for force_quiescent_state() to give you a ten-microsecond spike, so I am not surprised that you could not see the difference on 60 CPUs, which probably have given you something like 3 microseconds..
I will take a closer look at your patch (thank you for posting it!) -- but I still have not given up hope on the CPU-by-CPU approach.
Thanx, Paul
Quoted 168 lines
--- arch/x86/Kconfig | 4 ++++ kernel/rcutree.c | 49 ++++++++++++++++++++++++++++++++++++------------- kernel/rcutree.h | 16 ++++++++++++++++ 3 files changed, 56 insertions(+), 13 deletions(-)arch/x86/Kconfig +4
-0@@ -254,6 +254,10 @@ config ARCH_CPU_PROBE_RELEASE def_bool y depends on HOTPLUG_CPU && !XEN +config ARCH_RCU_FQS_LIMIT + int + default 16 + source "init/Kconfig" source "kernel/Kconfig.freezer"kernel/rcutree.c +36
-13@@ -1316,47 +1316,65 @@ void rcu_check_callbacks(int cpu, int us* have not yet encountered a quiescent state, using the function specified. * Also initiate boosting for any threads blocked on the root rcu_node. * +* Returns 0 if the scan could not be completed, 1 otherwise. * The caller must have suppressed start of new grace periods. */ -static void force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) +static int force_qs_rnp (struct rcu_state *rsp,int (*f)(struct rcu_data *)) {unsigned long bit;int cpu; +int cpusvisited =0 , done =1 ;unsigned long flags;unsigned long mask;struct rcu_node *rnp;rcu_for_each_leaf_node (rsp, rnp) { +if (rnp->grphi < rsp->fqs_next_cpu) +continue ; +if (rnp->grplo >rcu_get_max_cpu ()) +continue ; mask =0 ;raw_spin_lock_irqsave (&rnp->lock, flags);if (!rcu_gp_in_progress (rsp)) {raw_spin_unlock_irqrestore (&rnp->lock, flags); -return ; +return done; }if (rnp->qsmask ==0 ) {rcu_initiate_boost (rnp, flags);/* releases rnp->lock */ continue ; } - cpu = rnp->grplo; bit =1 ; -for (; cpu <= rnp->grphi; cpu++, bit <<=1 ) { -if ((rnp->qsmask & bit) !=0 && -f (per_cpu_ptr (rsp->rda, cpu))) + rsp->fqs_next_cpu = rnp->grphi +1 ; + cpusvisited += rnp->grphi - rnp->grplo; +for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, bit <<=1 ) { +if ((rnp->qsmask & bit) ==0 ) +continue ; +if (f (per_cpu_ptr (rsp->rda, cpu))) mask |= bit; } -if (mask !=0 ) { +if (mask !=0 ) {/* rcu_report_qs_rnp() releases rnp->lock. */ rcu_report_qs_rnp (mask, rsp, rnp, flags); -continue ; + }else +raw_spin_unlock_irqrestore (&rnp->lock, flags); + +if (cpusvisited >=FQS_MAX_CPU_VISIT ) { +if (FQS_MAX_CPU_VISIT !=NR_CPUS ) + done =0 ; +break ; } -raw_spin_unlock_irqrestore (&rnp->lock, flags); } rnp =rcu_get_root (rsp);if (rnp->qsmask ==0 ) {raw_spin_lock_irqsave (&rnp->lock, flags);rcu_initiate_boost (rnp, flags);/* releases rnp->lock. */ } + +if (done) + rsp->fqs_next_cpu =0 ; + +return done; }/* @@ -1366,6 +1384,7 @@ static void force_qs_rnp(struct rcu_statstatic void force_quiescent_state (struct rcu_state *rsp,int relaxed) {unsigned long flags; +int retval;struct rcu_node *rnp =rcu_get_root (rsp);if (!rcu_gp_in_progress (rsp))@@ -1398,21 +1417,25 @@ static void force_quiescent_state(structraw_spin_unlock (&rnp->lock);/* irqs remain disabled */ /* Record dyntick-idle state. */ -force_qs_rnp (rsp, dyntick_save_progress_counter); + retval =force_qs_rnp (rsp, dyntick_save_progress_counter);raw_spin_lock (&rnp->lock);/* irqs already disabled */ -if (rcu_gp_in_progress (rsp)) +if (rcu_gp_in_progress (rsp) && retval) { rsp->signaled =RCU_FORCE_QS ; + rsp->fqs_next_cpu =0 ; + }else if (rcu_gp_in_progress (rsp)) + rsp->jiffies_force_qs = jiffies -1 ;break ;case RCU_FORCE_QS :/* Check dyntick-idle state, send IPI to laggarts. */ raw_spin_unlock (&rnp->lock);/* irqs remain disabled */ -force_qs_rnp (rsp, rcu_implicit_dynticks_qs); + retval =force_qs_rnp (rsp, rcu_implicit_dynticks_qs);/* Leave state in case more forcing is required. */ -raw_spin_lock (&rnp->lock);/* irqs already disabled */ +if (rcu_gp_in_progress (rsp) && !retval) + rsp->jiffies_force_qs = jiffies -1 ;break ; } rsp->fqs_active =0 ;kernel/rcutree.h +16
-0@@ -354,6 +354,21 @@ do { \ }while (0 )/* +* Large latency-sensitive configurations can limit force_quiescent_state() +* latencies by defining an CONFIG_ARCH_RCU_FQS_LIMIT. This should be +* sized based on that architecture's cache-miss latency and the maximum +* desired force_quiescent_state latency. For example, if the cache-miss +* latency was 100 nanoseconds, and the maximum force_quiescent_state() +* latency contribution was 5 microseconds, then that architecture should +* define CONFIG_ARCH_RCU_FQS_LIMIT to be 50. + */ +#ifdef CONFIG_ARCH_RCU_FQS_LIMIT +#define FQS_MAX_CPU_VISIT CONFIG_ARCH_RCU_FQS_LIMIT +#else /* #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ +#define FQS_MAX_CPU_VISIT NR_CPUS +#endif /* #else #ifdef CONFIG_ARCH_RCU_FQS_LIMIT */ + +/* * RCU global state, including node hierarchy. This hierarchy is * represented in "heap" form in a dense array. The root (first level) * of the hierarchy is in ->node[0] (referenced by ->level[0]), the second @@ -391,6 +406,7 @@ struct rcu_state {/* starting new GP. */ raw_spinlock_t fqslock;/* Only one task forcing */ /* quiescent states. */ +int fqs_next_cpu;/* Next CPU for fqs to scan. */ unsigned long jiffies_force_qs;/* Time at which to invoke */ /* force_quiescent_state(). */ unsigned long n_force_qs;/* Number of calls to */
Quoted 1 line · Paul E. McKenney
Thank you very much for your efforts on this!!!
No problem, irq holdoff troubles are worth effort.
Quoted 6 lines
Given that you were seeing about 200-microsecond latency spikes on grace-period initialization, I would expect that you would need about 200 dyntick-idle CPUs for
force_quiescent_state()to give you a ten-microsecond spike, so I am not surprised that you could not see the difference on 60 CPUs, which probably have given you something like 3 microseconds..
Crawling over fewer locks should still save cycles, so I'll measure. Big box rt needs every little usec we can scrape together.
-Mike