linux-kernel - RE: [patch] x86, perf_counter, bts: add bts to perf

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <928CFBE8E7CB0040959E56B4EA41A77EC1BFF0A0@irsmsx504.ger.corp.intel.com>
Date:	Fri, 7 Aug 2009 13:18:13 +0100
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter

>-----Original Message-----
>From: Peter Zijlstra [mailto:a.p.zijlstra@...llo.nl]
>Sent: Friday, August 07, 2009 12:29 PM
>To: Metzger, Markus T
>Cc: Ingo Molnar; tglx@...utronix.de; hpa@...or.com; markus.t.metzger@...il.com; linux-
>kernel@...r.kernel.org
>Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter
>
>On Fri, 2009-08-07 at 11:18 +0100, Metzger, Markus T wrote:
>> >-----Original Message-----
>> >From: Peter Zijlstra [mailto:a.p.zijlstra@...llo.nl]
>> >Sent: Friday, August 07, 2009 10:21 AM
>> >To: Metzger, Markus T
>> >Cc: Ingo Molnar; tglx@...utronix.de; hpa@...or.com; markus.t.metzger@...il..com; linux-
>> >kernel@...r.kernel.org
>> >Subject: RE: [patch] x86, perf_counter, bts: add bts to perf_counter
>> >
>> >On Fri, 2009-08-07 at 08:29 +0100, Metzger, Markus T wrote:
>> >
>> >> I incorporated Peter's review comments, except that I would not enforce sample_period == 1
>> >> when branch tracing is requested. There might be users who want to sample the IP every 100.000'th
>> >> branch for profiling reasons.
>> >
>> >But in case you don't set sample_period==1 then you won't be able to
>> >match the BTS counter:
>> >
>> >+       if (unlikely((event ==
>> >+                     x86_pmu.event_map(PERF_COUNT_HW_BRANCH_INSTRUCTIONS)) &&
>> >+                    (hwc->sample_period == 1)))
>> >+               return X86_PMC_IDX_FIXED_BTS;
>> >
>> >Also,
>> >
>> >+               /*
>> >+                * Try to use BTS for branch tracing. If that is not
>> >+                * available, try to get a generic counter.
>> >+                */
>> >+               if (unlikely(!cpuc->ds))
>> >+                       goto try_generic;
>> >
>> >How will be use a generic counter for BTS, will it generate an NMI for
>> >every encountered branch? That might very will hit the throttle as that
>> >might be many.
>> >
>> >Would it not be better to force sample_period==1 usage onto the BTS and
>> >simply fail if its not available?
>>
>> In case someone requests a bigger sample-period, we would use the normal counter - as we
>> do without this patch.
>> We would also use the normal counter in case BTS is not available.
>>
>> In that case, we won't have BTS, we will have normal performance monitoring.
>> It will be throttled just like any other sampling request with small sample_period.
>>
>> If we forced sample_period = 1 for PERF_COUNT_HW_BRANCH_INSTRUCTIONS, we would remove
>> functionality.
>
>Right, what I'm worried about though is the BTS overload scenario.
>Normally when we'd create more counters than we'd have hardware for we'd
>simply time share the stuff.
>
>However BTS now has a second class fallback for period==1 which
>complicates all this because it will likely not generate consistent
>results.
>
>So I was thinking that _if_ the hardware supports BTS we'd not do the
>fallback to generic bits if event == HW_BRANCH_INST && period == 1.
>
>I agree on the period > 1 using the generic counters.


OK, that makes sense.

So I'll still check for sample_period=1 but when I fail to acquire BTS, I don't
fall back to the generic counter and return an error instead.

Is that it?


thanks,
markus.
---------------------------------------------------------------------
Intel GmbH
Dornacher Strasse 1
85622 Feldkirchen/Muenchen Germany
Sitz der Gesellschaft: Feldkirchen bei Muenchen
Geschaeftsfuehrer: Douglas Lusk, Peter Gleissner, Hannes Schwaderer
Registergericht: Muenchen HRB 47456 Ust.-IdNr.
VAT Registration No.: DE129385895
Citibank Frankfurt (BLZ 502 109 00) 600119052

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
by others is strictly prohibited. If you are not the intended
recipient, please contact the sender and delete all copies.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/