linux-kernel - Re: [RFC][PATCH 4/7] perf: Fix scaling vs enable_on

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Message-ID: <20160223154849.GC9102@krava.redhat.com>
Date:	Tue, 23 Feb 2016 16:48:49 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, alexander.shishkin@...ux.intel.com,
	eranian@...gle.com, linux-kernel@...r.kernel.org, vince@...ter.net,
	dvyukov@...gle.com, andi@...stfloor.org, sasha.levin@...cle.com,
	oleg@...hat.com, Pratyush Anand <panand@...hat.com>
Subject: Re: [RFC][PATCH 4/7] perf: Fix scaling vs enable_on_exec

On Tue, Feb 23, 2016 at 04:27:29PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 19, 2016 at 03:37:47PM +0100, Peter Zijlstra wrote:
> > Oleg reported that enable_on_exec results in weird scale factors.
> > 
> > The recent commit 3e349507d12d ("perf: Fix perf_enable_on_exec() event
> > scheduling") caused this by moving task_ctx_sched_out() from before
> > __perf_event_mask_enable() to after it.
> > 
> > The overlooked concequence of that change is that task_ctx_sched_out()
> > would update the ctx time fields, and now __perf_event_mask_enable()
> > uses stale time.
> > 
> > Fix this by adding an explicit time update.
> > 
> > While looking at this, I also found that we need an ctx->is_active
> > check in perf_install_in_context().
> > 
> > XXX: does this actually fix the reported issue? I'm not sure what the
> > reproduction case is. Also an earlier version made Jiri's machine
> > explode -- something I've not managed to reproduce either.
> 
> Jiri, can you have a look at this and perhaps share the reproducer?

yep, I'm testing this patchset, but got stuck with 'crash' tool to get
some reasonable output.. got stuck on unrelated sched deadlock ;-)

the reproducer is described in this email:
http://marc.info/?l=linux-kernel&m=145568006709552&w=2

CC-ing Pratyush

jjirka