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Message-ID: <20210616125452.GE801071@lothringen>
Date: Wed, 16 Jun 2021 14:54:52 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "hasegawa-hitomi@...itsu.com" <hasegawa-hitomi@...itsu.com>,
"'mingo@...nel.org'" <mingo@...nel.org>,
"'fweisbec@...il.com'" <fweisbec@...il.com>,
"'tglx@...utronix.de'" <tglx@...utronix.de>,
"'juri.lelli@...hat.com'" <juri.lelli@...hat.com>,
"'vincent.guittot@...aro.org'" <vincent.guittot@...aro.org>,
"'dietmar.eggemann@....com'" <dietmar.eggemann@....com>,
"'rostedt@...dmis.org'" <rostedt@...dmis.org>,
"'bsegall@...gle.com'" <bsegall@...gle.com>,
"'mgorman@...e.de'" <mgorman@...e.de>,
"'bristot@...hat.com'" <bristot@...hat.com>,
"'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: Utime and stime are less when getrusage (RUSAGE_THREAD) is
executed on a tickless CPU.
On Wed, May 19, 2021 at 11:24:58AM +0200, Peter Zijlstra wrote:
> On Wed, May 19, 2021 at 06:30:36AM +0000, hasegawa-hitomi@...itsu.com wrote:
> > Hi Ingo, Peter, Juri, and Vincent.
> >
> >
> > > Your email is malformed.
> >
> > I'm sorry. I was sent in the wrong format. I correct it and resend.
> > Thank you, Peter, for pointing this out.
> >
> >
> > I found that when I run getrusage(RUSAGE_THREAD) on a tickless CPU,
> > the utime and stime I get are less than the actual time, unlike when I run
> > getrusage(RUSAGE_SELF) on a single thread.
> > This problem seems to be caused by the fact that se.sum_exec_runtime is not
> > updated just before getting the information from 'current'.
> > In the current implementation, task_cputime_adjusted() calls task_cputime() to
> > get the 'current' utime and stime, then calls cputime_adjust() to adjust the
> > sum of utime and stime to be equal to cputime.sum_exec_runtime. On a tickless
> > CPU, sum_exec_runtime is not updated periodically, so there seems to be a
> > discrepancy with the actual time.
> > Therefore, I think I should include a process to update se.sum_exec_runtime
> > just before getting the information from 'current' (as in other processes
> > except RUSAGE_THREAD). I'm thinking of the following improvement.
> >
> > @@ void getrusage(struct task_struct *p, int who, struct rusage *r)
> > if (who == RUSAGE_THREAD) {
> > + task_sched_runtime(current);
> > task_cputime_adjusted(current, &utime, &stime);
> >
> > Is there any possible problem with this?
>
> Would be superfluous for CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y
> architectures at the very least.
>
> It also doesn't help any of the other callers, like for example procfs.
>
> Something like the below ought to work and fix all variants I think. But
> it does make the call significantly more expensive.
>
> Looking at thread_group_cputime() that already does something like this,
> but that's also susceptible to a variant of this very same issue; since
> it doesn't call it unconditionally, nor on all tasks, so if current
> isn't part of the threadgroup and/or another task is on a nohz_full cpu,
> things will go wobbly again.
>
> There's a note about syscall performance there, so clearly someone seems
> to care about that aspect of things, but it does suck for nohz_full.
>
> Frederic, didn't we have remote ticks that should help with this stuff?
>
> And mostly I think the trade-off here is that if you run on nohz_full,
> you're not expected to go do syscalls anyway (because they're sodding
> expensive) and hence the accuracy of these sort of things is mostly
> irrelevant.
>
> So it might be the use-case is just fundamentally bonkers and we
> shouldn't really bother fixing this.
>
> Anyway?
>
> ---
> diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
> index 872e481d5098..620871c8e4f8 100644
> --- a/kernel/sched/cputime.c
> +++ b/kernel/sched/cputime.c
> @@ -612,7 +612,7 @@ void cputime_adjust(struct task_cputime *curr, struct prev_cputime *prev,
> void task_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
> {
> struct task_cputime cputime = {
> - .sum_exec_runtime = p->se.sum_exec_runtime,
> + .sum_exec_runtime = task_sched_runtime(p),
> };
>
> task_cputime(p, &cputime.utime, &cputime.stime);
If necessary I guess we can do something like the below, which
would only add the overhead where it's required:
diff --git a/include/linux/sched/cputime.h b/include/linux/sched/cputime.h
index 6c9f19a33865..ce3c58286062 100644
--- a/include/linux/sched/cputime.h
+++ b/include/linux/sched/cputime.h
@@ -18,15 +18,16 @@
#endif /* CONFIG_VIRT_CPU_ACCOUNTING_NATIVE */
#ifdef CONFIG_VIRT_CPU_ACCOUNTING_GEN
-extern void task_cputime(struct task_struct *t,
+extern bool task_cputime(struct task_struct *t,
u64 *utime, u64 *stime);
extern u64 task_gtime(struct task_struct *t);
#else
-static inline void task_cputime(struct task_struct *t,
+static inline bool task_cputime(struct task_struct *t,
u64 *utime, u64 *stime)
{
*utime = t->utime;
*stime = t->stime;
+ return false;
}
static inline u64 task_gtime(struct task_struct *t)
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 872e481d5098..9392aea1804e 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -615,7 +615,8 @@ void task_cputime_adjusted(struct task_struct *p, u64 *ut, u64 *st)
.sum_exec_runtime = p->se.sum_exec_runtime,
};
- task_cputime(p, &cputime.utime, &cputime.stime);
+ if (task_cputime(p, &cputime.utime, &cputime.stime))
+ cputime.sum_exec_runtime = task_sched_runtime(p);
cputime_adjust(&cputime, &p->prev_cputime, ut, st);
}
EXPORT_SYMBOL_GPL(task_cputime_adjusted);
@@ -828,19 +829,21 @@ u64 task_gtime(struct task_struct *t)
* add up the pending nohz execution time since the last
* cputime snapshot.
*/
-void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
+bool task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
{
struct vtime *vtime = &t->vtime;
unsigned int seq;
u64 delta;
+ int ret;
if (!vtime_accounting_enabled()) {
*utime = t->utime;
*stime = t->stime;
- return;
+ return false;
}
do {
+ ret = false;
seq = read_seqcount_begin(&vtime->seqcount);
*utime = t->utime;
@@ -850,6 +853,7 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
if (vtime->state < VTIME_SYS)
continue;
+ ret = true;
delta = vtime_delta(vtime);
/*
@@ -861,6 +865,8 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
else
*utime += vtime->utime + delta;
} while (read_seqcount_retry(&vtime->seqcount, seq));
+
+ return ret;
}
static int vtime_state_fetch(struct vtime *vtime, int cpu)
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