linux-kernel - Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter constraint

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: <20150522134811.GI3644@twins.programming.kicks-ass.net>
Date:	Fri, 22 May 2015 15:48:11 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Vince Weaver <vincent.weaver@...ne.edu>,
	Jiri Olsa <jolsa@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 02/10] perf/x86: Improve HT workaround GP counter
 constraint

On Fri, May 22, 2015 at 06:40:49AM -0700, Stephane Eranian wrote:
> On Fri, May 22, 2015 at 6:36 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Fri, May 22, 2015 at 06:29:47AM -0700, Stephane Eranian wrote:
> >> On Fri, May 22, 2015 at 6:25 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >> > On Fri, May 22, 2015 at 06:07:00AM -0700, Stephane Eranian wrote:
> >> >>
> >> >> One other thing I noticed is that the --n_excl needs to be protected by the
> >> >> excl_cntrs->lock in put_excl_constraints().
> >> >
> >> > Nah, its strictly per cpu.
> >>
> >> No. the excl_cntrs struct is pointed to by cpuc but it is shared between the
> >> sibling HT. Otherwise this would not work!
> >
> > n_excl is per cpuc, see the trickery with has_exclusive vs
> > exclusive_present on how I avoid the lock.
> 
> Yes, but I believe  you create a store forward penalty with this.
> You store 16bits and you load 32 bits on the same cache line.

The store and load are fairly well spaced -- the entire scheduling fast
path is in between.

And such a penalty is still cheap compared to locking, no?
--
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/