Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755878Ab1ELJOy (ORCPT ); Thu, 12 May 2011 05:14:54 -0400 Received: from mail4.comsite.net ([205.238.176.238]:30013 "EHLO mail4.comsite.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755740Ab1ELJOw (ORCPT ); Thu, 12 May 2011 05:14:52 -0400 X-Default-Received-SPF: pass (skip=forwardok (res=PASS)) x-ip-name=71.22.127.106; Subject: [PATCH] seqlock: don't smp_rmb in seqlock reader spin loop From: Milton Miller To: Andrew Morton , Nick Piggin , Benjamin Herrenschmidt , Anton Blanchard , Thomas Gleixner , Eric Dumazet Cc: , , Ingo Molnar , Linus Torvalds , Andi Kleen Subject: [PATCH] seqlock: don't smp_rmb in seqlock reader spin loop Message-Id: In-Reply-To: References: <1304574244.32152.666.camel@edumazet-laptop> <1304576495.2943.40.camel@work-vm> <1304604284.3032.78.camel@edumazet-laptop> <1304608095.3032.95.camel@edumazet-laptop> <20110505210118.GI2925@one.firstfloor.org> <20110506165913.GF11636@one.firstfloor.org> <1304703767.3066.211.camel@edumazet-laptop> <20110506175051.GL11636@one.firstfloor.org> <1304710003.2821.0.camel@edumazet-laptop> <1304712267.2821.29.camel@edumazet-laptop> <1304713443.20980.124.camel@work-vm> <1304721004.2821.148.camel@edumazet-laptop> <1304722000.20980.130.camel@work-vm> <1304722835.2821.192.camel@edumazet-laptop> <1304724519.20980.139.camel@work-vm> Date: Thu, 12 May 2011 04:13:54 -0500 X-Originating-IP: 71.22.127.106 X-HeloNotChecked: Helo response was not checked before commands sent Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2533 Lines: 69 Move the smp_rmb after cpu_relax loop in read_seqlock and add ACCESS_ONCE to make sure the test and return are consistent. A multi-threaded core in the lab didn't like the update from 2.6.35 to 2.6.36, to the point it would hang during boot when multiple threads were active. Bisection showed af5ab277ded04bd9bc6b048c5a2f0e7d70ef0867 (clockevents: Remove the per cpu tick skew) as the culprit and it is supported with stack traces showing xtime_lock waits including tick_do_update_jiffies64 and/or update_vsyscall. Experimentation showed the combination of cpu_relax and smp_rmb was significantly slowing the progress of other threads sharing the core, and this patch is effective in avoiding the hang. A theory is the rmb is affecting the whole core while the cpu_relax is causing a resource rebalance flush, together they cause an interfernce cadance that is unbroken when the seqlock reader has interrupts disabled. At first I was confused why the refactor in 3c22cd5709e8143444a6d08682a87f4c57902df3 (kernel: optimise seqlock) didn't affect this patch application, but after some study that affected seqcount not seqlock. The new seqcount was not factored back into the seqlock. I defer that the future. While the removal of the timer interrupt offset created contention for the xtime lock while a cpu does the additonal work to update the system clock, the seqlock implementation with the tight rmb spin loop goes back much further, and is just waiting for the right trigger. Cc: Signed-off-by: Milton Miller --- To the readers of [RFC] time: xtime_lock is held too long: I initially thought x86 would not see this because rmb would be a nop, but upon closer inspection X86_PPRO_FENCE will add a lfence for rmb. milton Index: common/include/linux/seqlock.h =================================================================== --- common.orig/include/linux/seqlock.h 2011-04-06 03:27:02.000000000 -0500 +++ common/include/linux/seqlock.h 2011-04-06 03:35:02.000000000 -0500 @@ -88,12 +88,12 @@ static __always_inline unsigned read_seq unsigned ret; repeat: - ret = sl->sequence; - smp_rmb(); + ret = ACCESS_ONCE(sl->sequence); if (unlikely(ret & 1)) { cpu_relax(); goto repeat; } + smp_rmb(); return ret; } -- 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/