From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f46.google.com (mail-wm0-f46.google.com [74.125.82.46]) by kanga.kvack.org (Postfix) with ESMTP id B30FD6B0005 for ; Wed, 20 Jan 2016 16:28:10 -0500 (EST) Received: by mail-wm0-f46.google.com with SMTP id b14so50591888wmb.1 for ; Wed, 20 Jan 2016 13:28:10 -0800 (PST) Received: from mail-wm0-f41.google.com (mail-wm0-f41.google.com. [74.125.82.41]) by mx.google.com with ESMTPS id uc9si55990678wjc.194.2016.01.20.13.28.09 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 20 Jan 2016 13:28:09 -0800 (PST) Received: by mail-wm0-f41.google.com with SMTP id 123so149898748wmz.0 for ; Wed, 20 Jan 2016 13:28:09 -0800 (PST) Date: Wed, 20 Jan 2016 22:28:07 +0100 From: Michal Hocko Subject: Re: mm, vmstat: kernel BUG at mm/vmstat.c:1408! Message-ID: <20160120212806.GA26965@dhcp22.suse.cz> References: <5674A5C3.1050504@oracle.com> <20160120143719.GF14187@dhcp22.suse.cz> <569FA01A.4070200@oracle.com> <20160120151007.GG14187@dhcp22.suse.cz> <569FAC90.5030407@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: Sasha Levin , LKML , "linux-mm@kvack.org" , Andrew Morton On Wed 20-01-16 09:55:22, Christoph Lameter wrote: [...] > Subject: vmstat: Remove BUG_ON from vmstat_update > > If we detect that there is nothing to do just set the flag and do not check > if it was already set before. Races really do not matter. If the flag is > set by any code then the shepherd will start dealing with the situation > and reenable the vmstat workers when necessary again. > > Concurrent actions could be onlining and offlining of processors or be a > result of concurrency issues when updating the cpumask from multiple > processors. Now that 7e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle) is merged the VM_BUG_ON is simply bogus because vmstat_update might "race" with quiet_vmstat. The changelog should reflect that. What about the following wording? " Since 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle") quiet_vmstat might update cpu_stat_off and mark a particular cpu to be handled by vmstat_shepherd. This might trigger a VM_BUG_ON in vmstat_update because the work item might have been sleeping during the idle period and see the cpu_stat_off updated after the wake up. The VM_BUG_ON is therefore misleading and no more appropriate. Moreover it doesn't really suite any protection from real bugs because vmstat_shepherd will simply reschedule the vmstat_work anytime it sees a particular cpu set or vmstat_update would do the same from the worker context directly. Even when the two would race the result wouldn't be incorrect as the counters update is fully idempotent. Fixes: 0eb77e988032 ("vmstat: make vmstat_updater deferrable again and shut down on idle") CC: stable # 4.4+ " > Signed-off-by: Christoph Lameter > > Index: linux/mm/vmstat.c > =================================================================== > --- linux.orig/mm/vmstat.c > +++ linux/mm/vmstat.c > @@ -1408,17 +1408,7 @@ static void vmstat_update(struct work_st > * Defer the checking for differentials to the > * shepherd thread on a different processor. > */ > - int r; > - /* > - * Shepherd work thread does not race since it never > - * changes the bit if its zero but the cpu > - * online / off line code may race if > - * worker threads are still allowed during > - * shutdown / startup. > - */ > - r = cpumask_test_and_set_cpu(smp_processor_id(), > - cpu_stat_off); > - VM_BUG_ON(r); > + cpumask_set_cpu(smp_processor_id(), cpu_stat_off); > } > } -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org