linux-kernel - Re: [tip:perf/urgent] perf, x86: Catch spurious interrupts after disabling counters

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Message-ID: <20100929133923.GI13563@erda.amd.com>
Date:	Wed, 29 Sep 2010 15:39:23 +0200
From:	Robert Richter <robert.richter@....com>
To:	Stephane Eranian <eranian@...gle.com>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	"andi@...stfloor.org" <andi@...stfloor.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"gorcunov@...il.com" <gorcunov@...il.com>,
	"ying.huang@...el.com" <ying.huang@...el.com>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"ming.m.lin@...el.com" <ming.m.lin@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"dzickus@...hat.com" <dzickus@...hat.com>,
	"mingo@...e.hu" <mingo@...e.hu>
Subject: Re: [tip:perf/urgent] perf, x86: Catch spurious interrupts after
 disabling counters

On 29.09.10 09:13:30, Stephane Eranian wrote:

> for_each_set_bit(bit, (unsigned long *)&status, X86_PMC_IDX_MAX) {
>                 struct perf_event *event = cpuc->events[bit];
> 
>                 handled++;
> 
>                 if (!test_bit(bit, cpuc->active_mask))

			/* spurious interrupt here */

>                     continue;
> }
> 
> I think the logic is similar. What makes the difference, it seems, is that
> handled is incremented unconditionally if the ovfl_mask says it has
> an overflow, i.e., before active_mask is checked.

Note that we can use here for_each_set_bit() since we have the status
mask. So we may increment handled always.

On AMD we use for_each_counter(), but only check active counters to
avoid unnecessary rdmsrl()s for unused counters. But here, we only can
increment handled if we detect an overflow or if we know a counter was
disabled.

> On Westmere, we've seen situations where the overflow mask and active
> mask did not agree.

It's the 'spurious interrupt' branch above.

> On counter disable, the overflow mask bit is not cleared, thus one may iterate
> in the loop and fail the active_mask. But handled would be incremented in that
> case, so that would behave like in your patch.

Right, spurious interrupts are counted and a 'handled' is returned.

-Robert

-- 
Advanced Micro Devices, Inc.
Operating System Research Center

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