From: Johannes Weiner <hannes@cmpxchg.org> To: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 3/3] mm/sched: memdelay: memory health interface for systems and workloads Date: Mon, 31 Jul 2017 16:38:40 -0400 [thread overview] Message-ID: <20170731203839.GA5162@cmpxchg.org> (raw) In-Reply-To: <1501530579.9118.43.camel@gmx.de> On Mon, Jul 31, 2017 at 09:49:39PM +0200, Mike Galbraith wrote: > On Mon, 2017-07-31 at 14:41 -0400, Johannes Weiner wrote: > > > > Adding an rq counter for tasks inside memdelay sections should be > > straight-forward as well (except for maybe the migration cost of that > > state between CPUs in ttwu that Mike pointed out). > > What I pointed out should be easily eliminated (zero use case). How so? > > That leaves the question of how to track these numbers per cgroup at > > an acceptable cost. The idea for a tree of cgroups is that walltime > > impact of delays at each level is reported for all tasks at or below > > that level. E.g. a leave group aggregates the state of its own tasks, > > the root/system aggregates the state of all tasks in the system; hence > > the propagation of the task state counters up the hierarchy. > > The crux of the biscuit is where exactly the investment return lies. > Gathering of these numbers ain't gonna be free, no matter how hard you > try, and you're plugging into paths where every cycle added is made of > userspace hide. Right. But how to implement it sanely and optimize for cycles, and whether we want to default-enable this interface are two separate conversations. It makes sense to me to first make the implementation as lightweight on cycles and maintainability as possible, and then worry about the cost / benefit defaults of the shipped Linux kernel afterwards. That goes for the purely informative userspace interface, anyway. The easily-provoked thrashing livelock I have described in the email to Andrew is a different matter. If the OOM killer requires hooking up to this metric to fix it, it won't be optional. But the OOM code isn't part of this series yet, so again a conversation best had later, IMO. PS: I'm stealing the "made of userspace hide" thing.
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From: Johannes Weiner <hannes@cmpxchg.org> To: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Rik van Riel <riel@redhat.com>, Mel Gorman <mgorman@suse.de>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH 3/3] mm/sched: memdelay: memory health interface for systems and workloads Date: Mon, 31 Jul 2017 16:38:40 -0400 [thread overview] Message-ID: <20170731203839.GA5162@cmpxchg.org> (raw) In-Reply-To: <1501530579.9118.43.camel@gmx.de> On Mon, Jul 31, 2017 at 09:49:39PM +0200, Mike Galbraith wrote: > On Mon, 2017-07-31 at 14:41 -0400, Johannes Weiner wrote: > > > > Adding an rq counter for tasks inside memdelay sections should be > > straight-forward as well (except for maybe the migration cost of that > > state between CPUs in ttwu that Mike pointed out). > > What I pointed out should be easily eliminated (zero use case). How so? > > That leaves the question of how to track these numbers per cgroup at > > an acceptable cost. The idea for a tree of cgroups is that walltime > > impact of delays at each level is reported for all tasks at or below > > that level. E.g. a leave group aggregates the state of its own tasks, > > the root/system aggregates the state of all tasks in the system; hence > > the propagation of the task state counters up the hierarchy. > > The crux of the biscuit is where exactly the investment return lies. > Gathering of these numbers ain't gonna be free, no matter how hard you > try, and you're plugging into paths where every cycle added is made of > userspace hide. Right. But how to implement it sanely and optimize for cycles, and whether we want to default-enable this interface are two separate conversations. It makes sense to me to first make the implementation as lightweight on cycles and maintainability as possible, and then worry about the cost / benefit defaults of the shipped Linux kernel afterwards. That goes for the purely informative userspace interface, anyway. The easily-provoked thrashing livelock I have described in the email to Andrew is a different matter. If the OOM killer requires hooking up to this metric to fix it, it won't be optional. But the OOM code isn't part of this series yet, so again a conversation best had later, IMO. PS: I'm stealing the "made of userspace hide" thing. -- 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: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2017-07-31 20:38 UTC|newest] Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-07-27 15:30 [PATCH 0/3] memdelay: memory health metric for systems and workloads Johannes Weiner 2017-07-27 15:30 ` Johannes Weiner 2017-07-27 15:30 ` [PATCH 1/3] sched/loadavg: consolidate LOAD_INT, LOAD_FRAC macros Johannes Weiner 2017-07-27 15:30 ` Johannes Weiner 2017-07-27 15:30 ` [PATCH 2/3] mm: workingset: tell cache transitions from workingset thrashing Johannes Weiner 2017-07-27 15:30 ` Johannes Weiner 2017-07-27 15:30 ` [PATCH 3/3] mm/sched: memdelay: memory health interface for systems and workloads Johannes Weiner 2017-07-27 15:30 ` Johannes Weiner 2017-07-27 15:56 ` Johannes Weiner 2017-07-27 15:56 ` Johannes Weiner 2017-07-29 9:10 ` Peter Zijlstra 2017-07-29 9:10 ` Peter Zijlstra 2017-07-30 15:28 ` Johannes Weiner 2017-07-30 15:28 ` Johannes Weiner 2017-07-31 8:31 ` Peter Zijlstra 2017-07-31 8:31 ` Peter Zijlstra 2017-07-31 18:41 ` Johannes Weiner 2017-07-31 18:41 ` Johannes Weiner 2017-07-31 19:49 ` Mike Galbraith 2017-07-31 19:49 ` Mike Galbraith 2017-07-31 20:38 ` Johannes Weiner [this message] 2017-07-31 20:38 ` Johannes Weiner 2017-08-01 2:23 ` Mike Galbraith 2017-08-01 2:23 ` Mike Galbraith 2017-08-01 7:57 ` Peter Zijlstra 2017-08-01 7:57 ` Peter Zijlstra 2017-08-01 12:26 ` Johannes Weiner 2017-08-01 12:26 ` Johannes Weiner 2017-08-13 14:52 ` Peter Zijlstra 2017-08-13 14:52 ` Peter Zijlstra 2017-07-29 13:31 ` kbuild test robot 2017-07-27 20:43 ` [PATCH 0/3] memdelay: memory health metric " Andrew Morton 2017-07-27 20:43 ` Andrew Morton 2017-07-28 19:43 ` Johannes Weiner 2017-07-28 19:43 ` Johannes Weiner 2017-08-02 8:11 ` Michal Hocko 2017-08-02 8:11 ` Michal Hocko 2017-07-29 2:48 ` Mike Galbraith 2017-07-29 2:48 ` Mike Galbraith 2017-07-29 3:21 ` Mike Galbraith 2017-07-29 3:21 ` Mike Galbraith 2017-07-29 6:38 ` Mike Galbraith 2017-07-29 6:38 ` Mike Galbraith
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