linux-kernel - Re: Problem with perf hardware counters grouping

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: <1314895312.1485.2.camel@twins>
Date:	Thu, 01 Sep 2011 18:41:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vweaver1@...s.utk.edu>
Cc:	Mike Hommey <mh@...ndium.org>, linux-kernel@...r.kernel.org
Subject: Re: Problem with perf hardware counters grouping

On Thu, 2011-09-01 at 11:21 -0400, Vince Weaver wrote:
> On Thu, 1 Sep 2011, Peter Zijlstra wrote:
> 
> > What happens with your >3 case is that while the group is valid and
> > could fit on the PMU, it won't fit at runtime because the NMI watchdog
> > is taking one and won't budge (cpu-pinned counter have precedence over
> > any other kind), effectively starving your group of pmu runtime.

> UGH!  I just noticed this problem yesterday and was meaning to track it 
> down.
> 
> This obviously causes PAPI to fail if you try to use the maximum number of 
> counters.  Instead of getting EINVAL at open time or even at start time, 
> you just silently read all zeros at read time, and by then it's too late 
> to do anything useful about the problem because you just missed measuring 
> what you were trying to.
> 
> Is there any good workaround, or do we have to fall back to trying to 
> start/read/stop every proposed event set to make sure it's valid?

I guess my first question is going to be, how do you know what the
maximum number of counters is in the first place?


> This is going to seriously impact performance, and perf_event performance 
> is pretty bad to begin with.  The whole reason I was writing the tests to 
> trigger this is because PAPI users are complaining that perf_event 
> overhead is roughly twice that of perfctr or perfmon2, which I've verified 
> experimentally.

Yeah, you keep saying this, where does it come from? Only the lack of
userspace rdpmc?
--
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/