dubious process system time.
PATCH 1/1 dubious process system time.
From: Martin Schwidefsky <[email protected]>
[patch] dubious process system time.
The system time that is accounted to a process includes the time spent in three different contexts: normal system time, hardirq time and softirq time. To account hardirq time and sortirq time to a process seems wrong, because the process could just happen to run when the interrupt arrives that was caused by an i/o for a completly different process. And the sum over stime and cstime of all processes won't match cputstat->system either.
The following patch changes the accounting of system time so that hardirq and softirq time are not accounted to a process anymore.
---
kernel/sched.c | 7 +++---- 1 files changed, 3 insertions(+), 4 deletions(-)
kernel/sched.c +3 -4
@@ -2939,17 +2939,16 @@ void account_system_time(struct task_strstruct rq *rq =this_rq ();cputime64_t tmp; - p->stime =cputime_add (p->stime, cputime); -/* Add system time to cpustat. */ tmp =cputime_to_cputime64 (cputime);if (hardirq_count () - hardirq_offset) cpustat->irq =cputime64_add (cpustat->irq, tmp);else if (softirq_count ()) cpustat->softirq =cputime64_add (cpustat->softirq, tmp); -else if (p != rq->idle) +else if (p != rq->idle) { + p->stime =cputime_add (p->stime, cputime); cpustat->system =cputime64_add (cpustat->system, tmp); -else if (atomic_read (&rq->nr_iowait) >0 ) + }else if (atomic_read (&rq->nr_iowait) >0 ) cpustat->iowait =cputime64_add (cpustat->iowait, tmp);else cpustat->idle =cputime64_add (cpustat->idle, tmp);
Quoted 4 lines · Martin Schwidefsky
From: Martin Schwidefsky <[email protected]>
[patch] dubious process system time.
Quoted 9 lines
The system time that is accounted to a process includes the time spent in three different contexts: normal system time, hardirq time and softirq time. To account hardirq time and sortirq time to a process seems wrong, because the process could just happen to run when the interrupt arrives that was caused by an i/o for a completly different process. And the sum over stime and cstime of all processes won't match
cputstat->systemeither.
The following patch changes the accounting of system time so that hardirq and softirq time are not accounted to a process anymore.
So where does it get accounted then? It has to be accounted somewhere. Sounds like a quite radical change to me, might break a lot of existing assumptions.
-Andi
Quoted 10 lines · Andi Kleen
Quoted 9 lines
The system time that is accounted to a process includes the time spent in three different contexts: normal system time, hardirq time and softirq time. To account hardirq time and sortirq time to a process seems wrong, because the process could just happen to run when the interrupt arrives that was caused by an i/o for a completly different process. And the sum over stime and cstime of all processes won't match
cputstat->systemeither.
The following patch changes the accounting of system time so that hardirq and softirq time are not accounted to a process anymore.
Quoted 3 lines
So where does it get accounted then? It has to be accounted somewhere. Sounds like a quite radical change to me, might break a lot of existing assumptions.
At the moment hardirq+softirq is just added to a random process, in general this is completely wrong. You just need a system with a cpu hog and an i/o bound process and you get queer results.
To add hardirq+softirq to a single process is wrong to begin with, for that you would need to be able to identify the process that caused the i/o. And if two processes require a single file page then what? Split the time required to load the page to two processes? Not really. The conclusion is that hardirq+softirq time should not be accouted to any process. It is accounted globally in cpustat->softirq and cpustat->hardirq.
There is one assumption that would break by the change: that the sum of the hardirq and softirq time is contained in the sum of the stime and cstime fields of all processes. I don't think that this is relevant.
Signature · 8 lines
blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.
Quoted 3 lines
At the moment hardirq+softirq is just added to a random process, in general this is completely wrong.
It's better than not accounting it at all.
Quoted 2 lines
You just need a system with a cpu hog
and an i/o bound process and you get queer results.
Yes, but system load that is invisible to standard monitoring tools is even worse.
If you stop accounting it to random processes you have to account it somewhere else. Preferably somewhere that standard tools automatically pick up.
-Andi
Quoted 4 lines · Andi Kleen
Quoted 2 lines
At the moment hardirq+softirq is just added to a random process, in general this is completely wrong.
It's better than not accounting it at all.
I think it is worse than not accounting it. You are "charging" a process of some user for something that the user has nothing to do with.
Quoted 5 lines
Quoted 2 lines
You just need a system with a cpu hog
and an i/o bound process and you get queer results.Yes, but system load that is invisible to standard monitoring tools is even worse.
But it isn't invisible. cpustat->hardirq and cpustate->softirq will be increased. /proc/stat will show the system time spent in these two contexts.
Quoted 3 lines
If you stop accounting it to random processes you have to account it somewhere else. Preferably somewhere that standard tools automatically pick up.
Again, why do I have to account non-process related time to a process? Ihmo that is completly wrong.
Signature · 8 lines
blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.
Quoted 23 lines · Martin Schwidefsky
Quoted 5 lines · Andi Kleen
Quoted 3 lines
At the moment hardirq+softirq is just added to a random process, in general this is completely wrong.
It's better than not accounting it at all.
I think it is worse than not accounting it. You are "charging" a process of some user for something that the user has nothing to do with.
Quoted 5 lines
Quoted 2 lines
You just need a system with a cpu hog and an i/o bound process and you get queer results.
Yes, but system load that is invisible to standard monitoring tools is even worse.
But it isn't invisible.
cpustat->hardirqandcpustate->softirqwill be increased./proc/statwill show the system time spent in these two contexts.
Quoted 7 lines
Quoted 3 lines
If you stop accounting it to random processes you have to account it somewhere else. Preferably somewhere that standard tools automatically pick up.
Again, why do I have to account non-process related time to a process? Ihmo that is completly wrong.
If softirq time have to be accounted to a process (so as to not get lost), how about accounting it to the softirqd process? Much more reasonable than random processes.
Helge Hafting
Quoted 6 lines · Helge Hafting
Quoted 3 lines
Again, why do I have to account non-process related time to a process? Ihmo that is completly wrong.
If softirq time have to be accounted to a process (so as to not get lost), how about accounting it to the softirqd process? Much more reasonable than random processes.
The main question still is if it is correct to add softirq/hardirq time to the system time of a process. If the answer turns out to be yes, then it might be a clever idea to account softirq time to the softirqd. That still leaves the question what to do with hardirq time ..
My take still is that softirq/hardirq time does not belong to the system time of any process.
Signature · 8 lines
blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.
Quoted 6 lines · Martin Schwidefsky
The main question still is if it is correct to add softirq/hardirq time to the system time of a process. If the answer turns out to be yes, then it might be a clever idea to account softirq time to the softirqd. That still leaves the question what to do with hardirq time ..
My take still is that softirq/hardirq time does not belong to the system time of any process.
I agree.
Paul.
Quoted 3 lines · Martin Schwidefsky
The system time that is accounted to a process includes the time spent in three different contexts: normal system time, hardirq time and softirq time.
Is that true (at the moment) with CONFIG_VIRT_CPU_ACCOUNTING=y? I thought it wasn't.
Paul.
Quoted 6 lines · Paul Mackerras
Quoted 3 lines
The system time that is accounted to a process includes the time spent in three different contexts: normal system time, hardirq time and softirq time.
Is that true (at the moment) with
CONFIG_VIRT_CPU_ACCOUNTING=y? I thought it wasn't.
CONFIG_VIRT_CPU_ACCOUNTING improves the precision of the numbers that get accounted with account_[user,system,steal]_time. Which bucket the time goes into is decided in the three functions.
Signature · 8 lines
blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.