linux-kernel - RE: [tip:perf/core] perf: Add pmu specific data for perf task context

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Date:	Wed, 9 Dec 2015 15:25:25 +0000
From:	"Liang, Kan" <kan.liang@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"acme@...nel.org" <acme@...nel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:perf/core] perf: Add pmu specific data for perf task
 context


> 
> On Wed, Dec 09, 2015 at 02:59:21PM +0000, Liang, Kan wrote:
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c index
> > > 36babfd..97aa610 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -3508,11 +3515,6 @@ retry:
> > >  		if (!ctx)
> > >  			goto errout;
> > >
> > > -		if (task_ctx_data) {
> > > -			ctx->task_ctx_data = task_ctx_data;
> > > -			task_ctx_data = NULL;
> > > -		}
> > > -
> > >  		err = 0;
> > >  		mutex_lock(&task->perf_event_mutex);
> > >  		/*
> > > @@ -3526,6 +3528,10 @@ retry:
> > >  		else {
> > >  			get_ctx(ctx);
> > >  			++ctx->pin_count;
> > > +			if (task_ctx_data) {
> > > +				ctx->task_ctx_data = task_ctx_data;
> > > +				task_ctx_data = NULL;
> > > +			}
> > >  			rcu_assign_pointer(task->perf_event_ctxp[ctxn],
> > > ctx);
> > >  		}
> > >  		mutex_unlock(&task->perf_event_mutex);
> > >
> > >
> > > Does that make sense? No point in setting task_ctx_data if we're
> > > going to free the ctx and try again.
> >
> > The task_ctx_data will be checked before use. So it wouldn't crash the
> > system if it's NULL.
> 
> Yeah, I know, I checked :-)
> 
> > The problem is that LBR stack info will not be save/store on context
> > switch anymore. The user probably get wrong call stack information.
> 
> Yep
> 
> > May I know why you want to do that?
> 
> Because this seemed like a less fragile construct. When there's multiple
> event creations racing it seems possible (ableit entirely unlikely) to assign
> the allocated task_ctx_data to a ctx that we'll delete, and on the second go
> around re-allocate a ctx, but are left wihtout task_ctx_data to assign to it.
> 
> So by only assigning the task_ctx_data when we _know_ we've succeeded,
> we'll avoid this scenario.

Yes, I think it make sense to that.

Thanks,
Kan


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