linux-kernel - Re: [PATCH 0/4] perf: Intel uncore pmu counting support

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Date:	Mon, 04 Jul 2011 14:03:43 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/4] perf: Intel uncore pmu counting support

On Fri, 2011-07-01 at 18:49 +0800, Stephane Eranian wrote:
> On Fri, Jul 1, 2011 at 5:17 AM, Lin Ming <ming.m.lin@...el.com> wrote:
> > On Fri, 2011-07-01 at 00:27 +0800, Stephane Eranian wrote:
> >> On Thu, Jun 30, 2011 at 2:10 PM, Stephane Eranian <eranian@...gle.com> wrote:
> >> > On Thu, Jun 30, 2011 at 10:09 AM, Lin Ming <ming.m.lin@...el.com> wrote:
> >> >> Hi, all
> >> >>
> >> >> I posted uncore patches months ago, but it was pended due to an uncore
> >> >> interrupt problem.
> >> >>
> >> >> This series are cut to support uncore pmu counting only.
> >> >> So uncore interrupt handling is not needed.
> >> >>
> >> > You're making the assumption that when counting, you can never construct
> >> > a measurement that will cause a counter to overflow the 39 bits. If not, then
> >> > you need interrupt handling even when counting.
> >> >
> >> The actual counter width is 48. But wrmsrl() can only write the bottom 32 bits
> >> of a register. I think Intel fixed that only with SandyBridge (see Vol3b). Thus,
> >> the risk of 'silent' wrap around is much higher now as you have only 31 bits
> >> to play with.
> >
> > I just tested wrmsrl on uncore counters and it's surprised to me that it
> > supports full write.
> >
> >        val64.low  = 0xFFFFEEEE;
> >        val64.high = 0x12345678;
> >
> > On Nehalem/Westmere:
> >
> >        msr = 0x3b0; //NHM_MSR_UNCORE_PMC0
> >        wrmsrl(msr, val64.full & 0xfffffffffff); //48 bits counter
> >        rdmsrl(msr, val64.full);
> >        printfk("counter value: 0x%llx\n", val64.full);
> >
> > I got:
> >        counter value: 0x5678ffffeeee
> >
> > On SandyBridge:
> >
> >        msr = 0x716; //SNB_MSR_UNC_CBO_1_PER_CTR0
> >        wrmsrl(msr, val64.full & 0xfffffffffff); //44 bits counter
> >        rdmsrl(msr, val64.full);
> >        printfk("counter value: 0x%llx\n", val64.full);
> >
> > I got:
> >        counter value: 0x678ffffeeee
> >
> I tried on my Nehalem and SandyBridge (model 42) and I get the same results.
> The restriction applies only to core PMU counters then.
> 
> But what's so weird is that on SandyBridge the counter counters are still
> limited:
> 
> uncore counter value: 0x678ffffeeee
>    core counter value: 0xffffffffeeee
> 
> Yet, IA32_PERF_CAPABILITIES.FW_WIDTH (bit13) is set, thus wrmsrl()
> should write the full 48-bit width (see Vol3b 30.2.2.3).

No.

The SandyBridge uncore generic counters are 44 bits width and the fixed
counter is 48 bit width.

Lin Ming

> 
> Further testing revealed that you get full width ONLY with the fixed counter
> counters:
> 
> FW_WIDTH=1
> uncore counter value: 0x678ffffeeee
> core counter value: 0xffffffffeeee
> fixed counter value: 0x678ffffeeee
> 
> I suspect there is a bug in the HW or a bogus description in the SDM.
> I will check on that with Intel.
> 
> 
> >> But if I read your patch correctly, it seems you are avoiding wrmsrl() on the
> >> counter. Instead, you are reading it when you start (prev_count) and using
> >> that value to compute the delta on stop.
> >>
> >> Am I understanding your workaround correctly?
> >
> > Yes, but I didn't realize that it's a workaround.
> >
> > Lin Ming
> >
> >>
> >>
> >> >
> >> >> The uncore pmu type is allocated dynamically and exported via sysfs.
> >> >> $ cat /sys/bus/event_source/devices/uncore/type
> >> >> 6
> >> >>
> >> >> You can count uncore raw events as below,
> >> >> $ perf stat -e uncore:r0101 ls
> >> >>
> >> >> It reads uncore pmu type id from sysfs to setup perf_event_attr::type.
> >> >>
> >> >> Comments are appreciated.
> >> >>
> >> >> Thanks,
> >> >> Lin Ming
> >> >>
> >> >>
> >> >
> >
> >
> >


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