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

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Message-ID: <20160223163550.GC11724@dhcppc3.redhat.com>
Date:	Tue, 23 Feb 2016 22:05:50 +0530
From:	Pratyush Anand <panand@...hat.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, 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
Subject: Re: [RFC][PATCH 4/7] perf: Fix scaling vs enable_on_exec

Hi Peter/Jiri,

On 23/02/2016:04:48:49 PM, Jiri Olsa wrote:
> 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

Thanks for CCing.

Its better with this patch, still count is 1 more in case of higher probe hits (
like 65535 times).

Reproducer is:
---------------------------------------------------------
git clone https://github.com/rfmvh/perftool-testsuite.git
cd perftool-testsuite/base_probe/
./setup.sh 
for i in 1 2 3 103 997 65535; do
perf probe -x examples/exact_counts --add f_${i}x
done
perf stat -x';' -e 'probe_exact:*' examples/exact_counts
---------------------------------------------------------

I see following with above code:

65536;;probe_exact:f_65535x;84476560;100.00
997;;probe_exact:f_997x;84476560;100.00
103;;probe_exact:f_103x;84476560;100.00
3;;probe_exact:f_3x;84476560;100.00
2;;probe_exact:f_2x;84476560;100.00
1;;probe_exact:f_1x;84476560;100.00

~Pratyush

PS: Do I need to take all patches of series? Currently I have taken only this.