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From: Frederic Weisbecker <frederic@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Wanpeng Li <wanpengli@tencent.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Yauheni Kaliuta <yauheni.kaliuta@redhat.com>,
	Ingo Molnar <mingo@kernel.org>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH 04/25] vtime: Spare a seqcount lock/unlock cycle on context switch
Date: Wed, 25 Sep 2019 16:42:21 +0200	[thread overview]
Message-ID: <20190925144219.GA17854@lenoir> (raw)
In-Reply-To: <20181120132512.GQ2131@hirez.programming.kicks-ass.net>

Sorry to answer 10 month later. You certainly have lost track of this. I have.
I'm re-issuing this patchset but more piecewise to make the review easier.

In case you still care, I'm answering your comments below. But you can skip
that and wait for the new version that I'm about to post.


On Tue, Nov 20, 2018 at 02:25:12PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 14, 2018 at 03:45:48AM +0100, Frederic Weisbecker wrote:
> 
> So I definitely like avoiding that superfluous atomic op, however:
> 
> > @@ -730,19 +728,25 @@ static void vtime_account_guest(struct task_struct *tsk,
> >  	}
> >  }
> >  
> > +static void __vtime_account_kernel(struct task_struct *tsk,
> > +				   struct vtime *vtime)
> 
> Your last patch removed a __function, and now you're adding one back :/

Yes, in fact I removed a __function to avoid having two in the end.

I can't think of a better name. vtime_account_kernel_locked() maybe,
but it's not event locked, it's seqcount write.

> 
> >  {
> >  	/* We might have scheduled out from guest path */
> >  	if (tsk->flags & PF_VCPU)
> >  		vtime_account_guest(tsk, vtime);
> >  	else
> >  		vtime_account_system(tsk, vtime);
> > +}
> > +
> > +void vtime_account_kernel(struct task_struct *tsk)
> > +{
> > +	struct vtime *vtime = &tsk->vtime;
> > +
> > +	if (!vtime_delta(vtime))
> > +		return;
> > +
> 
> See here the fast path (is it worth it?)

Might be worth testing if that fast path is often hit indeed. With
any sensible clock we should at least have a few nanosecs to account.

> 
> > +	write_seqcount_begin(&vtime->seqcount);
> > +	__vtime_account_kernel(tsk, vtime);
> >  	write_seqcount_end(&vtime->seqcount);
> >  }
> 
> > +void vtime_task_switch_generic(struct task_struct *prev)
> >  {
> >  	struct vtime *vtime = &prev->vtime;
> 
> And observe a distinct lack of that same fast path..

Right but in any case we have to lock (seqcount) here
since at least vtime->state has to be set to VTIME_INACTIVE.

> 
> >  
> >  	write_seqcount_begin(&vtime->seqcount);
> > +	if (is_idle_task(prev))
> > +		vtime_account_idle(prev);
> > +	else
> > +		__vtime_account_kernel(prev, vtime);
> >  	vtime->state = VTIME_INACTIVE;
> >  	write_seqcount_end(&vtime->seqcount);
> >  
> > -- 
> > 2.7.4
> > 

  reply	other threads:[~2019-09-25 14:42 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-14  2:45 [PATCH 00/25] sched/nohz: Make kcpustat vtime aware (Fix kcpustat on nohz_full) Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 01/25] sched/vtime: Fix guest/system mis-accounting on task switch Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 02/25] sched/vtime: Protect idle accounting under vtime seqcount Frederic Weisbecker
2018-11-20 13:19   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 03/25] vtime: Rename vtime_account_system() to vtime_account_kernel() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 04/25] vtime: Spare a seqcount lock/unlock cycle on context switch Frederic Weisbecker
2018-11-20 13:25   ` Peter Zijlstra
2019-09-25 14:42     ` Frederic Weisbecker [this message]
2018-11-14  2:45 ` [PATCH 05/25] sched/vtime: Record CPU under seqcount for kcpustat needs Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 06/25] sched/cputime: Add vtime idle task state Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 07/25] sched/cputime: Add vtime guest " Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 08/25] vtime: Exit vtime before exit_notify() Frederic Weisbecker
2018-11-20 13:54   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 09/25] kcpustat: Track running task following vtime sequences Frederic Weisbecker
2018-11-20 13:58   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 10/25] context_tracking: Remove context_tracking_active() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 11/25] context_tracking: s/context_tracking_is_enabled/context_tracking_enabled() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 12/25] context_tracking: Rename context_tracking_is_cpu_enabled() to context_tracking_enabled_this_cpu() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 13/25] context_tracking: Introduce context_tracking_enabled_cpu() Frederic Weisbecker
2018-11-20 14:02   ` Peter Zijlstra
2018-11-14  2:45 ` [PATCH 14/25] sched/vtime: Rename vtime_accounting_cpu_enabled() to vtime_accounting_enabled_this_cpu() Frederic Weisbecker
2018-11-14  2:45 ` [PATCH 15/25] sched/vtime: Introduce vtime_accounting_enabled_cpu() Frederic Weisbecker
2018-11-20 14:04   ` Peter Zijlstra
2018-11-14  2:46 ` [PATCH 16/25] sched/cputime: Allow to pass cputime index on user/guest accounting Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 17/25] sched/cputime: Standardize the kcpustat index based accounting functions Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 18/25] vtime: Track nice-ness on top of context switch Frederic Weisbecker
2018-11-20 14:09   ` Peter Zijlstra
2018-11-14  2:46 ` [PATCH 19/25] sched/vite: Handle nice updates under vtime Frederic Weisbecker
2018-11-20 14:17   ` Peter Zijlstra
2018-11-26 15:53     ` Frederic Weisbecker
2018-11-26 16:11       ` Peter Zijlstra
2018-11-26 18:41         ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 20/25] sched/kcpustat: Introduce vtime-aware kcpustat accessor Frederic Weisbecker
2018-11-20 14:23   ` Peter Zijlstra
2018-11-20 22:40     ` Frederic Weisbecker
2018-11-21  8:18       ` Peter Zijlstra
2018-11-21  8:35         ` Peter Zijlstra
2018-11-21 16:33         ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 21/25] procfs: Use vtime aware " Frederic Weisbecker
2018-11-20 14:24   ` Peter Zijlstra
2018-11-20 22:31     ` Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 22/25] cpufreq: " Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 23/25] leds: Use vtime aware kcpustat accessors Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 24/25] rackmeter: " Frederic Weisbecker
2018-11-14  2:46 ` [PATCH 25/25] sched/vtime: Clarify vtime_task_switch() argument layout Frederic Weisbecker

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