linux-kernel - Re: [RFC PATCH] perf: Add load latency monitoring on Intel Nehalem/Westmere

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Message-ID: <AANLkTi==HfPX1J7yRW=9nbDJfY7Bd4=kZhphq+SoU+-k@mail.gmail.com>
Date:	Thu, 23 Dec 2010 12:05:30 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lin Ming <ming.m.lin@...el.com>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>, paulus <paulus@...ba.org>
Subject: Re: [RFC PATCH] perf: Add load latency monitoring on Intel Nehalem/Westmere

On Thu, Dec 23, 2010 at 11:48 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2010-12-23 at 11:31 +0100, Stephane Eranian wrote:
>> On Thu, Dec 23, 2010 at 11:18 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Thu, 2010-12-23 at 16:59 +0800, Lin Ming wrote:
>> >> > {L1, L2, L3, RAM}x{snoop, local, remote}x{shared, exclusive} + {unknown,
>> >> > uncached, IO}
>> >> >
>> >> > Which takes all of 5 bits to encode.
>> >>
>> >> Do you mean below encoding?
>> >>
>> >> bits4 3 2 1 0
>> >>     + + + + +
>> >>     | | | | |
>> >>     | | | {L1, L2, L3, RAM} or {unknown, uncached, IO}
>> >>     | | |
>> >>     | {snoop, local, remote, OTHER}
>> >>     |
>> >>     {shared, exclusive}
>> >>
>> >> If bits(2-3) is OTHER, then bits(0-1) is the encoding of {unknown,
>> >> uncached, IO}.
>> >
>> > That is most certainly a very valid encoding, and a rather nice one at
>> > that. I hadn't really gone further than: 4*3*2 + 3 < 2^5 :-)
>> >
>> > If you also make OTHER=0, then a valid encoding for unknown is also 0,
>> > which is a nice meaning for 0...
>> >
>> I am not sure how you would cover the 9 possibilities for data source as
>> shown in Table 10-13 using this encoding. Could you show me?
>
> Ah, I think I see the problem, there's multiple L3-snoops, I guess we
> can fix that by extending the {shared, exclusive} to full MESI, growing
> us to 6 bits.
>
> I'm assuming you mean "Table 30-13. Data Source Encoding for Load
> Latency Record", which has 14 values defined.
>
Yes.

> Value   Intel                           Perf
> 0x0     Unknown L3                      Unknown
>
> 0x1     L1                              L1-local
>
> 0x2     Pending core cache HIT          L2-snoop
>        Outstanding core cache miss to

Not clear how you know this is snoop or L2?

I suspect this one is saying you have a request for a line
for which there is already a pending request underway. Could
be the first came from prefetchers, the 2nd is actual demand.

Let me check with Intel. The table is unclear.

>        the same line was underway
> 0x3     L2                              L2-local
>
> 0x4     L3-snoop, no coherency actions  L3-snoop-I

I am not sure I understand what you mean by local vs. remote
in your terminology.


> 0x5     L3-snoop, found no M            L3-snoop-S
> 0x6     L3-snoop, found M               L3-snoop-M
>
> 0x8     L3-miss, snoop, shared          RAM-snoop-S
> 0xA     L3-miss, local, shared          RAM-local-S
> 0xB     L3-miss, remote, shared         RAM-remote-S
>
> 0xC     L3-miss, local, exclusive       RAM-local-E
> 0xD     L3-miss, remote, exclusive      RAM-remote-E
>
> 0xE     IO                              IO
> 0xF     uncached                        uncached
>
>
> Leaving us with:
>
> {L1, L2, L3, RAM}x{snoop, local, remote}x{modified, exclusive, shared, invalid} + {unknown, uncached, IO}
>
> Now the question is, is this sufficient to map all data sources from
> other archs as well?
>
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