sched,time: reduce nohz_full syscall overhead 40%
v4 cover sched,time: reduce nohz_full syscall overhead 40%
(v4: address comments by Peter and Frederic)
Running with nohz_full introduces a fair amount of overhead. Specifically, various things that are usually done from the timer interrupt are now done at syscall, irq, and guest entry and exit times.
However, some of the code that is called every single time has only ever worked at jiffy resolution. The code in __acct_update_integrals was also doing some unnecessary calculations.
Getting rid of the unnecessary calculations, without changing any of the functionality in __acct_update_integrals gets us about an 11% win.
Not calling the time statistics updating code more than once per jiffy, like is done on housekeeping CPUs and on all the CPUs of a non-nohz_full system, shaves off a further 30%.
I tested this series with a microbenchmark calling an invalid syscall number ten million times in a row, on a nohz_full cpu.
Run times for the microbenchmark:
4.4 3.8 seconds
4.5-rc1 3.7 seconds
4.5-rc1 + first patch 3.3 seconds
4.5-rc1 + first 3 patches 3.1 seconds
4.5-rc1 + all patches 2.3 seconds
PATCH v4 1/4 sched,time: remove non-power-of-two divides from __acct_update_integrals
From: Rik van Riel <[email protected]>
When running a microbenchmark calling an invalid syscall number in a loop, on a nohz_full CPU, we spend a full 9% of our CPU time in __acct_update_integrals.
This function converts cputime_t to jiffies, to a timeval, only to convert the timeval back to microseconds before discarding it.
This patch leaves __acct_update_integrals functionally equivalent, but speeds things up by about 12%, with 10 million calls to an invalid syscall number dropping from 3.7 to 3.25 seconds.
--- kernel/tsacct.c | 26 ++++++++++++++++---------- 1 file changed, 16 insertions(+), 10 deletions(-)
kernel/tsacct.c +16 -10
@@ -93,9 +93,11 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p) {struct mm_struct *mm; -/* convert pages-usec to Mbyte-usec */ - stats->coremem = p->acct_rss_mem1 *PAGE_SIZE /MB ; - stats->virtmem = p->acct_vm_mem1 *PAGE_SIZE /MB ; +/* convert pages-nsec/1024 to Mbyte-usec, see __acct_update_integrals */ + stats->coremem = p->acct_rss_mem1 *PAGE_SIZE ; +do_div (stats->coremem,1000 *KB ); + stats->virtmem = p->acct_vm_mem1 *PAGE_SIZE ; +do_div (stats->virtmem,1000 *KB ); mm =get_task_mm (p);if (mm) {/* adjust to KB unit */
@@ -125,22 +127,26 @@ static void __acct_update_integrals(struct task_struct *tsk, {if (likely (tsk->mm)) {cputime_t time, dtime; -struct timeval value;unsigned long flags;u64 delta;local_irq_save (flags); time = stime + utime; dtime = time - tsk->acct_timexpd; -jiffies_to_timeval (cputime_to_jiffies (dtime), &value); - delta = value.tv_sec; - delta = delta *USEC_PER_SEC + value.tv_usec; +/* Avoid division: cputime_t is often in nanoseconds already. */ + delta =cputime_to_nsecs (dtime); -if (delta ==0 ) +if (delta <TICK_NSEC ) goto out; + tsk->acct_timexpd = time; - tsk->acct_rss_mem1 += delta *get_mm_rss (tsk->mm); - tsk->acct_vm_mem1 += delta * tsk->mm->total_vm; +/* +* Divide by 1024 to avoid overflow, and to avoid division. +* The final unit reported to userspace is Mbyte-usecs, +* the rest of the math is done in xacct_add_tsk. +*/ + tsk->acct_rss_mem1 += delta *get_mm_rss (tsk->mm) >>10 ; + tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >>10 ; out:local_irq_restore (flags); }
Signature · 1 line
2.5.0
PATCH v4 2/4 acct,time: change indentation in __acct_update_integrals
In reply to [email protected]From: Rik van Riel <[email protected]>
Change the indentation in __acct_update_integrals to make the function a little easier to read.
--- kernel/tsacct.c | 51 ++++++++++++++++++++++++++------------------------- 1 file changed, 26 insertions(+), 25 deletions(-)
kernel/tsacct.c +26 -25
@@ -125,31 +125,32 @@ void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)static void __acct_update_integrals (struct task_struct *tsk,cputime_t utime,cputime_t stime) { -if (likely (tsk->mm)) { -cputime_t time, dtime; -unsigned long flags; -u64 delta; - -local_irq_save (flags); - time = stime + utime; - dtime = time - tsk->acct_timexpd; -/* Avoid division: cputime_t is often in nanoseconds already. */ - delta =cputime_to_nsecs (dtime); - -if (delta <TICK_NSEC ) - goto out; - - tsk->acct_timexpd = time; -/* -* Divide by 1024 to avoid overflow, and to avoid division. -* The final unit reported to userspace is Mbyte-usecs, -* the rest of the math is done in xacct_add_tsk. -*/ - tsk->acct_rss_mem1 += delta *get_mm_rss (tsk->mm) >>10 ; - tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >>10 ; - out: -local_irq_restore (flags); - } +cputime_t time, dtime; +unsigned long flags; +u64 delta; + +if (!likely (tsk->mm)) +return ; + +local_irq_save (flags); + time = stime + utime; + dtime = time - tsk->acct_timexpd; +/* Avoid division: cputime_t is often in nanoseconds already. */ + delta =cputime_to_nsecs (dtime); + +if (delta <TICK_NSEC ) + goto out; + + tsk->acct_timexpd = time; +/* +* Divide by 1024 to avoid overflow, and to avoid division. +* The final unit reported to userspace is Mbyte-usecs, +* the rest of the math is done in xacct_add_tsk. +*/ + tsk->acct_rss_mem1 += delta *get_mm_rss (tsk->mm) >>10 ; + tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >>10 ; +out: +local_irq_restore (flags); }/**
Signature · 1 line
2.5.0
PATCH v4 3/4 time,acct: drop irq save & restore from __acct_update_integrals
In reply to [email protected]From: Rik van Riel <[email protected]>
It looks like all the call paths that lead to __acct_update_integrals already have irqs disabled, and __acct_update_integrals does not need to disable irqs itself.
This is very convenient since about half the CPU time left in this function was spent in local_irq_save alone.
Performance of a microbenchmark that calls an invalid syscall ten million times in a row on a nohz_full CPU improves 21% vs. 4.5-rc1 with both the removal of divisions from __acct_update_integrals and this patch, with runtime dropping from 3.7 to 2.9 seconds.
With these patches applied, the highest remaining cpu user in the trace is native_sched_clock, which is addressed in the next patch.
--- kernel/tsacct.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)
kernel/tsacct.c +4 -5
@@ -126,20 +126,18 @@ static void __acct_update_integrals(struct task_struct *tsk, cputime_t utime, cputime_t stime) {cputime_t time, dtime; -unsigned long flags;u64 delta;if (!likely (tsk->mm))return ; -local_irq_save (flags); time = stime + utime; dtime = time - tsk->acct_timexpd;/* Avoid division: cputime_t is often in nanoseconds already. */ delta =cputime_to_nsecs (dtime);if (delta <TICK_NSEC ) - goto out; +return ; tsk->acct_timexpd = time;/*
@@ -149,8 +147,6 @@ static void __acct_update_integrals(struct task_struct *tsk,*/ tsk->acct_rss_mem1 += delta *get_mm_rss (tsk->mm) >>10 ; tsk->acct_vm_mem1 += delta * tsk->mm->total_vm >>10 ; -out: -local_irq_restore (flags); }/**
@@ -160,9 +156,12 @@ static void __acct_update_integrals(struct task_struct *tsk,void acct_update_integrals (struct task_struct *tsk) {cputime_t utime, stime; +unsigned long flags; +local_irq_save (flags);task_cputime (tsk, &utime, &stime);__acct_update_integrals (tsk, utime, stime); +local_irq_restore (flags); }/**
Signature · 1 line
2.5.0
PATCH v4 4/4 sched,time: only call account_{user,sys,guest,idle}_time once a jiffy
In reply to [email protected]From: Rik van Riel <[email protected]>
After removing __acct_update_integrals from the profile, native_sched_clock remains as the top CPU user. This can be reduced by only calling account_{user,sys,guest,idle}_time once per jiffy for long running tasks on nohz_full CPUs.
This will reduce timing accuracy on nohz_full CPUs to jiffy based sampling, just like on normal CPUs. It results in totally removing native_sched_clock from the profile, and significantly speeding up the syscall entry and exit path, as well as irq entry and exit, and kvm guest entry & exit.
This code relies on another CPU advancing jiffies when the system is busy. On a nohz_full system, this is done by a housekeeping CPU.
A microbenchmark calling an invalid syscall number 10 million times in a row speeds up an additional 30% over the numbers with just the previous patches, for a total speedup of about 40% over 4.4 and 4.5-rc1.
Run times for the microbenchmark:
4.4 3.8 seconds
4.5-rc1 3.7 seconds
4.5-rc1 + first patch 3.3 seconds
4.5-rc1 + first 3 patches 3.1 seconds
4.5-rc1 + all patches 2.3 seconds
--- include/linux/sched.h | 1 + kernel/sched/cputime.c | 35 +++++++++++++++++++++++++++++------ 2 files changed, 30 insertions(+), 6 deletions(-)
include/linux/sched.h +1 -0
@@ -1532,6 +1532,7 @@ struct task_struct {struct prev_cputime prev_cputime;#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN seqcount_t vtime_seqcount; +unsigned long vtime_jiffies;unsigned long long vtime_snap;enum { /* Task is sleeping or running in a CPU with VTIME inactive */
kernel/sched/cputime.c +29 -6
@@ -668,6 +668,15 @@ void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime#endif /* !CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */ #ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN +static bool vtime_jiffies_changed (struct task_struct *tsk,unsigned long now) +{ +if (tsk->vtime_jiffies == jiffies) +return false; + + tsk->vtime_jiffies = jiffies; +return true; +} +static unsigned long long vtime_delta (struct task_struct *tsk) {unsigned long long clock;
@@ -699,6 +708,9 @@ static void __vtime_account_system(struct task_struct *tsk)void vtime_account_system (struct task_struct *tsk) { +if (!vtime_jiffies_changed (tsk, jiffies)) +return ; +write_seqcount_begin (&tsk->vtime_seqcount);__vtime_account_system (tsk);write_seqcount_end (&tsk->vtime_seqcount);
@@ -707,7 +719,8 @@ void vtime_account_system(struct task_struct *tsk)void vtime_gen_account_irq_exit (struct task_struct *tsk) {write_seqcount_begin (&tsk->vtime_seqcount); -__vtime_account_system (tsk); +if (vtime_jiffies_changed (tsk, jiffies)) +__vtime_account_system (tsk);if (context_tracking_in_user ()) tsk->vtime_snap_whence =VTIME_USER ;write_seqcount_end (&tsk->vtime_seqcount);
@@ -718,16 +731,19 @@ void vtime_account_user(struct task_struct *tsk)cputime_t delta_cpu;write_seqcount_begin (&tsk->vtime_seqcount); - delta_cpu =get_vtime_delta (tsk); tsk->vtime_snap_whence =VTIME_SYS ; -account_user_time (tsk, delta_cpu,cputime_to_scaled (delta_cpu)); +if (vtime_jiffies_changed (tsk, jiffies)) { + delta_cpu =get_vtime_delta (tsk); +account_user_time (tsk, delta_cpu,cputime_to_scaled (delta_cpu)); + }write_seqcount_end (&tsk->vtime_seqcount); }void vtime_user_enter (struct task_struct *tsk) {write_seqcount_begin (&tsk->vtime_seqcount); -__vtime_account_system (tsk); +if (vtime_jiffies_changed (tsk, jiffies)) +__vtime_account_system (tsk); tsk->vtime_snap_whence =VTIME_USER ;write_seqcount_end (&tsk->vtime_seqcount); }
@@ -742,7 +758,8 @@ void vtime_guest_enter(struct task_struct *tsk)* that can thus safely catch up with a tickless delta. */ write_seqcount_begin (&tsk->vtime_seqcount); -__vtime_account_system (tsk); +if (vtime_jiffies_changed (tsk, jiffies)) +__vtime_account_system (tsk); current->flags |=PF_VCPU ;write_seqcount_end (&tsk->vtime_seqcount); }
@@ -759,8 +776,12 @@ EXPORT_SYMBOL_GPL(vtime_guest_exit);void vtime_account_idle (struct task_struct *tsk) { -cputime_t delta_cpu =get_vtime_delta (tsk); +cputime_t delta_cpu; + +if (!vtime_jiffies_changed (tsk, jiffies)) +return ; + delta_cpu =get_vtime_delta (tsk);account_idle_time (delta_cpu); }
@@ -773,6 +794,7 @@ void arch_vtime_task_switch(struct task_struct *prev)write_seqcount_begin (¤t->vtime_seqcount); current->vtime_snap_whence =VTIME_SYS ; current->vtime_snap =sched_clock_cpu (smp_processor_id ()); + current->vtime_jiffies = jiffies;write_seqcount_end (¤t->vtime_seqcount); }
@@ -784,6 +806,7 @@ void vtime_init_idle(struct task_struct *t, int cpu)write_seqcount_begin (&t->vtime_seqcount); t->vtime_snap_whence =VTIME_SYS ; t->vtime_snap =sched_clock_cpu (cpu); + t->vtime_jiffies = jiffies;write_seqcount_end (&t->vtime_seqcount);local_irq_restore (flags); }
Signature · 1 line
2.5.0
Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy
kernel/sched/cputime.c void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime · line 678
Quoted 12 lines · On Mon, 2016-02-01 at 14:21 -0500, [email protected] wrote:
From: Rik van Riel <[email protected]>
#ifdef
CONFIG_VIRT_CPU_ACCOUNTING_GENkernel/sched/cputime.c +8
-0+static bool vtime_jiffies_changed (struct task_struct *tsk,unsigned long now) +{ +if (tsk->vtime_jiffies == jiffies) +return false; + + tsk->vtime_jiffies = jiffies; +return true; +}
I am guessing you intended to use 'now' instead of jiffies ?
Re: [PATCH 4/4] sched,time: only call account_{user,sys,guest,idle}_time once a jiffy
Quoted 4 lines · Eric Dumazet
Quoted 2 lines · On Mon, 2016-02-01 at 14:21 -0500, [email protected] wrote:
From: Rik van Riel <[email protected]>
Quoted 11 lines
kernel/sched/cputime.c void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut, cputime · line 678
Quoted 9 lines
#ifdef
CONFIG_VIRT_CPU_ACCOUNTING_GENkernel/sched/cputime.c +8
-0+static bool vtime_jiffies_changed (struct task_struct *tsk,unsigned long now) +{ +if (tsk->vtime_jiffies == jiffies) +return false; + + tsk->vtime_jiffies = jiffies; +return true; +}I am guessing you intended to use 'now' instead of jiffies ?
Yes indeed. Good catch.
I have fixed this up in my local tree, and will wait for more people's comments before sending out a v5 (tomorrow?).
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