Re: [PATCH 09/10] s390/cputime: delayed accounting of system time

Martin Schwidefsky
2016-12-12T10:28:07Z

On Sat, 10 Dec 2016 02:48:06 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Tue, Dec 06, 2016 at 03:32:22AM +0100, Frederic Weisbecker wrote:
> > From: Martin Schwidefsky <schwidefsky@de.ibm.com>
> >
> > The account_system_time() function is called with a cputime that
> > occurred while running in the kernel. The function detects which
> > context the CPU is currently running in and accounts the time to
> > the correct bucket. This forces the arch code to account the
> > cputime for hardirq and softirq immediately.
> >
> > Such accounting function can be costly and perform unwelcome divisions
> > and multiplications, among others.
> >
> > The arch code can delay the accounting for system time. For s390
> > the accounting is done once per timer tick and for each task switch.
> >
> > Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Tony Luck <tony.luck@intel.com>
> > Cc: Fenghua Yu <fenghua.yu@intel.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Stanislaw Gruszka <sgruszka@redhat.com>
> > Cc: Wanpeng Li <wanpeng.li@hotmail.com>
> > [rebase against latest cputime tree, massaged changelog accordingly]
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
>
> Looking at this patch again, I think I need to do another pass on it.
> Comments below:
>
> > /*
> > * Update process times based on virtual cpu times stored by entry.S
> > * to the lowcore fields user_timer, system_timer & steal_clock.
> > */
> > static int do_account_vtime(struct task_struct *tsk, int hardirq_offset)
> > {
> > - u64 timer, clock, user, system, steal;
> > - u64 user_scaled, system_scaled;
> > + u64 timer, clock, user, guest, system, hardirq, softirq, steal;
> >
> > timer = S390_lowcore.last_update_timer;
> > clock = S390_lowcore.last_update_clock;
> > @@ -110,36 +119,57 @@ static int do_account_vtime(struct task_struct *tsk, int hardirq_offset)
> > #endif
> > : "=m" (S390_lowcore.last_update_timer),
> > "=m" (S390_lowcore.last_update_clock));
> > - S390_lowcore.system_timer += timer - S390_lowcore.last_update_timer;
> > - S390_lowcore.steal_timer += S390_lowcore.last_update_clock - clock;
> > + clock = S390_lowcore.last_update_clock - clock;
> > + timer -= S390_lowcore.last_update_timer;
> > +
> > + if ((tsk->flags & PF_VCPU) && (irq_count() - hardirq_offset == 0))
> > + S390_lowcore.guest_timer += timer;
> > + else if (hardirq_count() - hardirq_offset)
> > + S390_lowcore.hardirq_timer += timer;
>
> We should get rid of the hardirq_offset argument, it doesn't really make sense
> anymore. Also it makes the accounting buggy now. It's called from the tick
> through account_user_time() with hardirq_offset=1, so the irq time is incorrectly
> accumulated as system time. Guest time may be incorrect too.
>
> In fact it may have been buggy even before this patchset because vtime_account_user()
> isn't only called from the tick but also from task switch, and hardirq_offset remains 1
> for those two cases. Not good.

For s390 the do_account_vtime function is called from vtime_task_switch and vtime_flush.
1) vtime_task_switch is exclusively called from finish_task_switch outside of irq context.
The call to do_account_vtime with hardirq_offset==0 from vtime_task_switch is correct.
2) The call to vtime_flush in vtime_common_task_switch is irrelevant for s390 as we
define __ARCH_HAS_VTIME_TASK_SWITCH
3) The call to vtime_flush in account_process_tick is done in irq context from
update_process_times. hardirq_offset==1 is also correct.

As far as s390 is concerned that looks good.

> > + else if (in_serving_softirq())
> > + S390_lowcore.softirq_timer += timer;
> > + else
> > + S390_lowcore.system_timer += timer;
> >
> > /* Update MT utilization calculation */
> > if (smp_cpu_mtid &&
> > time_after64(jiffies_64, this_cpu_read(mt_scaling_jiffies)))
> > update_mt_scaling();
> >
> > + /* Calculate cputime delta */
> > user = S390_lowcore.user_timer - tsk->thread.user_timer;
> > - S390_lowcore.steal_timer -= user;
> > tsk->thread.user_timer = S390_lowcore.user_timer;
> > -
> > + guest = S390_lowcore.guest_timer - tsk->thread.guest_timer;
> > + tsk->thread.guest_timer = S390_lowcore.guest_timer;
> > system = S390_lowcore.system_timer - tsk->thread.system_timer;
> > - S390_lowcore.steal_timer -= system;
> > tsk->thread.system_timer = S390_lowcore.system_timer;
> > + hardirq = S390_lowcore.hardirq_timer - tsk->thread.hardirq_timer;
> > + tsk->thread.hardirq_timer = S390_lowcore.hardirq_timer;
> > + softirq = S390_lowcore.softirq_timer - tsk->thread.softirq_timer;
> > + tsk->thread.softirq_timer = S390_lowcore.softirq_timer;
> > + S390_lowcore.steal_timer +=
> > + clock - user - guest - system - hardirq - softirq;
> >
> > - user_scaled = user;
> > - system_scaled = system;
> > - /* Do MT utilization scaling */
> > - if (smp_cpu_mtid) {
> > - u64 mult = __this_cpu_read(mt_scaling_mult);
> > - u64 div = __this_cpu_read(mt_scaling_div);
> > + /* Push account value */
> > + if (user) {
> > + account_user_time(tsk, user);
> > + tsk->utimescaled += scale_vtime(user);
> > + }
> >
> > - user_scaled = (user_scaled * mult) / div;
> > - system_scaled = (system_scaled * mult) / div;
> > + if (guest) {
> > + account_guest_time(tsk, guest);
> > + tsk->utimescaled += scale_vtime(guest);
> > }
> > - account_user_time(tsk, user);
> > - tsk->utimescaled += user_scaled;
> > - account_system_time(tsk, hardirq_offset, system);
> > - tsk->stimescaled += system_scaled;
> > +
> > + if (system)
> > + account_system_index_scaled(tsk, system, scale_vtime(system),
> > + CPUTIME_SYSTEM);
> > + if (hardirq)
> > + account_system_index_scaled(tsk, hardirq, scale_vtime(hardirq),
> > + CPUTIME_IRQ);
> > + if (softirq)
> > + account_system_index_scaled(tsk, softirq, scale_vtime(softirq),
> > + CPUTIME_SOFTIRQ);
> >
> > steal = S390_lowcore.steal_timer;
> > if ((s64) steal > 0) {
> > @@ -147,16 +177,22 @@ static int do_account_vtime(struct task_struct *tsk, int hardirq_offset)
> > account_steal_time(steal);
> > }
> >
> > - return virt_timer_forward(user + system);
> > + return virt_timer_forward(user + guest + system + hardirq + softirq);
> > }
> >
> > void vtime_task_switch(struct task_struct *prev)
> > {
> > do_account_vtime(prev, 0);
>
> This call should be removed, the task switch already calls vtime_account_user().

The vtime_account_user function is empty for s390..

> > prev->thread.user_timer = S390_lowcore.user_timer;
> > + prev->thread.guest_timer = S390_lowcore.guest_timer;
> > prev->thread.system_timer = S390_lowcore.system_timer;
> > + prev->thread.hardirq_timer = S390_lowcore.hardirq_timer;
> > + prev->thread.softirq_timer = S390_lowcore.softirq_timer;
> > S390_lowcore.user_timer = current->thread.user_timer;
> > + S390_lowcore.guest_timer = current->thread.guest_timer;
> > S390_lowcore.system_timer = current->thread.system_timer;
> > + S390_lowcore.hardirq_timer = current->thread.hardirq_timer;
> > + S390_lowcore.softirq_timer = current->thread.softirq_timer;
> > }
>

--
blue skies,
Martin.

"Reality continues to ruin my life." - Calvin.