From: Eric B Munson <emunson@mgebm.net>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: a.p.zijlstra@chello.nl, linux-kernel@vger.kernel.org,
paulus@samba.org, anton@samba.org, acme@ghostprotocols.net,
mingo@elte.hu, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks
Date: Wed, 30 Mar 2011 14:36:56 -0400 [thread overview]
Message-ID: <20110330183656.GA2564@mgebm.net> (raw)
In-Reply-To: <1301433165.2402.689.camel@pasglop>
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On Wed, 30 Mar 2011, Benjamin Herrenschmidt wrote:
> On Tue, 2011-03-29 at 10:25 -0400, Eric B Munson wrote:
> > Here I made the assumption that the hardware would never remove more events in
> > a speculative roll back than it had added. This is not a situation I
> > encoutered in my limited testing, so I didn't think underflow was possible. I
> > will send out a V2 using the signed 32 bit delta and remeber to CC stable
> > this time.
>
> I'm not thinking about underflow but rollover... or that isn't possible
> with those counters ? IE. They don't wrap back to 0 after hitting
> ffffffff ?
>
They do roll over to 0 after ffffffff, but I thought that case was already
covered by the perf_event_interrupt. Are you concerned that we will reset a
counter and speculative roll back will underflow that counter?
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WARNING: multiple messages have this Message-ID (diff)
From: Eric B Munson <emunson@mgebm.net>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: a.p.zijlstra@chello.nl, paulus@samba.org, mingo@elte.hu,
acme@ghostprotocols.net, linuxppc-dev@lists.ozlabs.org,
linux-kernel@vger.kernel.org, anton@samba.org
Subject: Re: [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks
Date: Wed, 30 Mar 2011 14:36:56 -0400 [thread overview]
Message-ID: <20110330183656.GA2564@mgebm.net> (raw)
In-Reply-To: <1301433165.2402.689.camel@pasglop>
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On Wed, 30 Mar 2011, Benjamin Herrenschmidt wrote:
> On Tue, 2011-03-29 at 10:25 -0400, Eric B Munson wrote:
> > Here I made the assumption that the hardware would never remove more events in
> > a speculative roll back than it had added. This is not a situation I
> > encoutered in my limited testing, so I didn't think underflow was possible. I
> > will send out a V2 using the signed 32 bit delta and remeber to CC stable
> > this time.
>
> I'm not thinking about underflow but rollover... or that isn't possible
> with those counters ? IE. They don't wrap back to 0 after hitting
> ffffffff ?
>
They do roll over to 0 after ffffffff, but I thought that case was already
covered by the perf_event_interrupt. Are you concerned that we will reset a
counter and speculative roll back will underflow that counter?
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next prev parent reply other threads:[~2011-03-30 18:37 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-03-25 13:28 [PATCH] POWER: perf_event: Skip updating kernel counters if register value shrinks Eric B Munson
2011-03-25 13:28 ` Eric B Munson
2011-03-29 6:03 ` Benjamin Herrenschmidt
2011-03-29 6:03 ` Benjamin Herrenschmidt
2011-03-29 14:25 ` Eric B Munson
2011-03-29 14:25 ` Eric B Munson
2011-03-29 21:12 ` Benjamin Herrenschmidt
2011-03-29 21:12 ` Benjamin Herrenschmidt
2011-03-30 18:36 ` Eric B Munson [this message]
2011-03-30 18:36 ` Eric B Munson
2011-03-31 6:04 ` Benjamin Herrenschmidt
2011-03-31 6:04 ` Benjamin Herrenschmidt
2011-03-31 16:14 ` Eric B Munson
2011-03-31 16:14 ` Eric B Munson
2011-04-06 21:27 ` Eric B Munson
2011-04-06 21:27 ` Eric B Munson
2011-04-07 4:22 ` Benjamin Herrenschmidt
2011-04-07 4:22 ` Benjamin Herrenschmidt
2011-04-07 16:16 ` Eric B Munson
2011-04-07 16:16 ` Eric B Munson
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