Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu barrier

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu barrier
Date	2015-11-17 13:30 +0100
Message-ID	<qvNyy-4XK-1@gated-at.bofh.it> (permalink)
References	<qpEeC-3hu-3@gated-at.bofh.it> <qvMsO-4ho-5@gated-at.bofh.it> <qvMsO-4ho-3@gated-at.bofh.it> <qvMCt-4n1-13@gated-at.bofh.it> <qvMMa-4qX-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

On Tue, Nov 17, 2015 at 05:07:38PM +0530, Vineet Gupta wrote:
> > as is ARC-SMP seems to have a _lot_ of superfluous
> > barriers many of which have no explanation yet (I'm thinking of those
> > extra smp_mb()s in the lock primitives).
> 
> Other than the lock primitives can u think of any more.

Not of the top of my head.

> I verified that with llock/scond based spinlocks, those smp_mb() can be safely
> removed.

Good!

> I didn't send that patch over yet as part of puzzle is why removing them
> in EX based locks causes hackbench to jitter on quad core builds. This required
> some perf investigation but that seems to be causing some sort of livelock with
> callgraph profiling which is what I'm debugging currently :-)

So there's two superfluous barriers right; the one before LOCK and the
one after UNLOCK. Does removing either cause the jitter? I'm thinking
that maybe its the smp_mb after UNLOCK that force flushes the store
buffer that helps (MIPS has something similar).

> BTW since we are on the topic we have this loop in stack unwinder which can
> potentially cause RCU stalls, actual lockups etc. I was planning to add the
> following - does that seem fine to you.

Worries me more than anything. How could you get stuck in there?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Thread

Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 12:20 +0100
  Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Peter Zijlstra <peterz@infradead.org> - 2015-11-17 12:30 +0100
    Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 12:40 +0100
      Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Peter Zijlstra <peterz@infradead.org> - 2015-11-17 13:30 +0100
        Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 13:40 +0100
          Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Peter Zijlstra <peterz@infradead.org> - 2015-11-17 13:50 +0100
            Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Vineet Gupta <Vineet.Gupta1@synopsys.com> - 2015-11-17 14:40 +0100
              Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu  barrier Peter Zijlstra <peterz@infradead.org> - 2015-11-17 15:00 +0100

csiph-web