Re: [rfc 1/3] perf, x86: P4 PMU - describe config format

Cyrill Gorcunov
2010-11-26T17:16:50Z

On Fri, Nov 26, 2010 at 05:22:51PM +0100, Stephane Eranian wrote:
> On Fri, Nov 26, 2010 at 4:27 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> > On Fri, Nov 26, 2010 at 02:54:39PM +0100, Stephane Eranian wrote:
> > ...
> >> >
> >> > No, single thread mode means _any_ single thread is running,
> >> > Stephane I'll describe some more a bit later (as only reach home),
> >> > ok?
> >>
> >> From the manual:
> >>
> >> 00 — None. Count only when neither logical processor is active.
> >> 01 — Single. Count only when one logical processor is active (either 0 or 1).
> >> 10 — Both. Count only when both logical processors are active.
> >> 11 — Any. Count when either logical processor is active.
> >>
> >> In per-thread mode, you won't hit 00. I suspect you want to
> >> disallow 01, 10 (or CAP_SYS_ADMIN). Otherwise, you want
> >> to force 11, i.e., can't figure out what's going on in the other
> >> HT thread.
> >>
> >
> >  No ;) The key moment here that this flags are related to _activity_ of
> > logical thread and I guess they were introduced just to allow measuring
> > if user-space application does win from using HT or not (since for
> > some loads the HT simply drops the perfomance).
> >
>
> I think what they call 'logical CPU' is what the kernel calls CPU.

yes

> So I think bits 16-17 are used if you want to measure on CPU0 only
> when CPU1 (assume both share the same physical core) is active
> or inactive or don't care. You're right that I believe this mode was
> introduced to measure the level of concurrency between HT
> thread (logical CPUs).

but "single thread mode" measures the event if *any* thread
is active at a time, if we would assume that your assumption
is right -- there would be no need for T1_OS/T1_USR.

So I must admit I'm confused, the Oprofile do things simplier --
it just sets "thread any" ;)

>
> In architectural perfmon the .any modifier is slightly different.
> It indicates whether you want to measure only yourself or both
> threads (regardless of the state of the other HT thread). In other
> words, it is not because .any=1 that the event counts ONLY when
> both threads (logical CPUs) are active.
>
...

Stephane, could you do a test? We need a pinned event which would be
running on say cpu0 only and if you set P4_CCCR_THREAD_SINGLE in CCCR
and event would be still incrementing -- this mean my theory is correct
and this "thread" field means "when" to count, if not -- you're right
and "thread" field means "where" instead.

Cyrill