From: Peter Zijlstra <peterz@infradead.org> To: Aaron Lu <aaron.lu@intel.com> Cc: Michal Hocko <mhocko@kernel.org>, Tim Chen <tim.c.chen@linux.intel.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>, Tim Chen <tim.c.chen@intel.com>, Andrew Morton <akpm@linux-foundation.org>, Ying Huang <ying.huang@intel.com> Subject: Re: [PATCH v2 0/5] mm: support parallel free of memory Date: Fri, 17 Mar 2017 14:16:50 +0100 [thread overview] Message-ID: <20170317131650.2xtsbh4rwd7qtzef@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <20170317123315.GA1929@aaronlu.sh.intel.com> On Fri, Mar 17, 2017 at 08:33:15PM +0800, Aaron Lu wrote: > On Fri, Mar 17, 2017 at 08:47:08AM +0100, Michal Hocko wrote: > > On Thu 16-03-17 11:36:21, Tim Chen wrote: > > [...] > > > Perhaps we can only do this expedited exit only when there are idle cpus around. > > > We can use the root sched domain's overload indicator for such a quick check. > > > > This is not so easy, I am afraid. Those CPUs might be idle for a good > > reason (power saving etc.). You will never know by simply checking > > Is it that those CPUs are deliberately put into idle mode to save power? No, forced idle injection is an abomination. > > one metric. This is why doing these optimistic parallelization > > optimizations is far from trivial. This is not the first time somebody > > wants to do this. People are trying to make THP migration faster > > doing the similar thing. I guess we really need a help from the > > scheduler to do this properly, though. I've been thinking about an API > > (e.g. try_to_run_in_backgroun) which would evaluate all these nasty > > details and either return with -EBUSY or kick the background thread to > > accomplish the work if the system is reasonably idle. I am not really > > sure whether such an API is viable though. Peter, what do you think? > > I would very much like to know what these nasty details are and what > 'reasonably idle' actually means, I think they are useful to understand > the problem and define the API. A CPU being idle doesn't mean it'll be idle long enough to do your additional work. The CPU not being idle affects scheduling latency. It also increases power usage and thermals. If your workload wants peak single threaded throughput, making the other CPUs do work will lower its turbo boost range for example. An 'obvious' solution that doesn't work is an idle scheduler; its an instant priority inversion if you take locks there. Not to mention you loose any fwd progress guarantees for any work you put in.
WARNING: multiple messages have this Message-ID (diff)
From: Peter Zijlstra <peterz@infradead.org> To: Aaron Lu <aaron.lu@intel.com> Cc: Michal Hocko <mhocko@kernel.org>, Tim Chen <tim.c.chen@linux.intel.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Dave Hansen <dave.hansen@intel.com>, Tim Chen <tim.c.chen@intel.com>, Andrew Morton <akpm@linux-foundation.org>, Ying Huang <ying.huang@intel.com> Subject: Re: [PATCH v2 0/5] mm: support parallel free of memory Date: Fri, 17 Mar 2017 14:16:50 +0100 [thread overview] Message-ID: <20170317131650.2xtsbh4rwd7qtzef@hirez.programming.kicks-ass.net> (raw) In-Reply-To: <20170317123315.GA1929@aaronlu.sh.intel.com> On Fri, Mar 17, 2017 at 08:33:15PM +0800, Aaron Lu wrote: > On Fri, Mar 17, 2017 at 08:47:08AM +0100, Michal Hocko wrote: > > On Thu 16-03-17 11:36:21, Tim Chen wrote: > > [...] > > > Perhaps we can only do this expedited exit only when there are idle cpus around. > > > We can use the root sched domain's overload indicator for such a quick check. > > > > This is not so easy, I am afraid. Those CPUs might be idle for a good > > reason (power saving etc.). You will never know by simply checking > > Is it that those CPUs are deliberately put into idle mode to save power? No, forced idle injection is an abomination. > > one metric. This is why doing these optimistic parallelization > > optimizations is far from trivial. This is not the first time somebody > > wants to do this. People are trying to make THP migration faster > > doing the similar thing. I guess we really need a help from the > > scheduler to do this properly, though. I've been thinking about an API > > (e.g. try_to_run_in_backgroun) which would evaluate all these nasty > > details and either return with -EBUSY or kick the background thread to > > accomplish the work if the system is reasonably idle. I am not really > > sure whether such an API is viable though. Peter, what do you think? > > I would very much like to know what these nasty details are and what > 'reasonably idle' actually means, I think they are useful to understand > the problem and define the API. A CPU being idle doesn't mean it'll be idle long enough to do your additional work. The CPU not being idle affects scheduling latency. It also increases power usage and thermals. If your workload wants peak single threaded throughput, making the other CPUs do work will lower its turbo boost range for example. An 'obvious' solution that doesn't work is an idle scheduler; its an instant priority inversion if you take locks there. Not to mention you loose any fwd progress guarantees for any work you put in. -- 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:[~2017-03-17 13:16 UTC|newest] Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top 2017-03-15 8:59 [PATCH v2 0/5] mm: support parallel free of memory Aaron Lu 2017-03-15 8:59 ` Aaron Lu 2017-03-15 9:00 ` [PATCH v2 1/5] mm: add tlb_flush_mmu_free_batches Aaron Lu 2017-03-15 9:00 ` Aaron Lu 2017-03-15 9:00 ` [PATCH v2 2/5] mm: parallel free pages Aaron Lu 2017-03-15 9:00 ` Aaron Lu 2017-03-15 9:42 ` Hillf Danton 2017-03-15 9:42 ` Hillf Danton 2017-03-15 11:54 ` Aaron Lu 2017-03-15 11:54 ` Aaron Lu 2017-03-15 9:00 ` [PATCH v2 3/5] mm: use a dedicated workqueue for the free workers Aaron Lu 2017-03-15 9:00 ` Aaron Lu 2017-03-22 6:33 ` Minchan Kim 2017-03-22 6:33 ` Minchan Kim 2017-03-22 8:41 ` Aaron Lu 2017-03-22 8:41 ` Aaron Lu 2017-03-22 8:55 ` Minchan Kim 2017-03-22 8:55 ` Minchan Kim 2017-03-22 13:43 ` Aaron Lu 2017-03-22 13:43 ` Aaron Lu 2017-03-23 5:53 ` Minchan Kim 2017-03-23 5:53 ` Minchan Kim 2017-03-23 15:38 ` Dave Hansen 2017-03-23 15:38 ` Dave Hansen 2017-03-24 12:37 ` Aaron Lu 2017-03-24 12:37 ` Aaron Lu 2017-03-15 9:00 ` [PATCH v2 4/5] mm: add force_free_pages in zap_pte_range Aaron Lu 2017-03-15 9:00 ` Aaron Lu 2017-03-15 9:00 ` [PATCH v2 5/5] mm: add debugfs interface for parallel free tuning Aaron Lu 2017-03-15 9:00 ` Aaron Lu 2017-03-15 14:18 ` [PATCH v2 0/5] mm: support parallel free of memory Michal Hocko 2017-03-15 14:18 ` Michal Hocko 2017-03-15 15:44 ` Aaron Lu 2017-03-15 15:44 ` Aaron Lu 2017-03-15 16:28 ` Michal Hocko 2017-03-15 16:28 ` Michal Hocko 2017-03-15 21:38 ` Tim Chen 2017-03-15 21:38 ` Tim Chen 2017-03-16 9:07 ` Michal Hocko 2017-03-16 9:07 ` Michal Hocko 2017-03-16 18:36 ` Tim Chen 2017-03-16 18:36 ` Tim Chen 2017-03-17 7:47 ` Michal Hocko 2017-03-17 7:47 ` Michal Hocko 2017-03-17 8:07 ` Minchan Kim 2017-03-17 8:07 ` Minchan Kim 2017-03-17 12:33 ` Aaron Lu 2017-03-17 12:33 ` Aaron Lu 2017-03-17 12:59 ` Michal Hocko 2017-03-17 12:59 ` Michal Hocko 2017-03-17 13:16 ` Peter Zijlstra [this message] 2017-03-17 13:16 ` Peter Zijlstra 2017-03-17 12:53 ` Peter Zijlstra 2017-03-17 12:53 ` Peter Zijlstra 2017-03-17 13:05 ` Michal Hocko 2017-03-17 13:05 ` Michal Hocko 2017-03-21 14:54 ` Dave Hansen 2017-03-21 14:54 ` Dave Hansen 2017-03-22 8:02 ` Aaron Lu 2017-03-22 8:02 ` Aaron Lu 2017-03-24 7:04 ` Aaron Lu 2017-03-24 7:04 ` Aaron Lu 2017-03-21 15:18 ` Tim Chen 2017-03-21 15:18 ` Tim Chen 2017-03-16 6:54 ` Aaron Lu 2017-03-16 6:54 ` Aaron Lu 2017-03-16 7:34 ` Aaron Lu 2017-03-16 7:34 ` Aaron Lu 2017-03-16 13:51 ` Aaron Lu 2017-03-16 13:51 ` Aaron Lu 2017-03-16 14:14 ` Aaron Lu 2017-03-16 14:14 ` Aaron Lu 2017-03-15 14:56 ` Vlastimil Babka 2017-03-15 14:56 ` Vlastimil Babka 2017-03-15 15:50 ` Aaron Lu 2017-03-15 15:50 ` Aaron Lu 2017-03-17 3:10 ` Aaron Lu 2017-03-17 3:10 ` Aaron Lu 2017-03-16 19:38 ` Alex Thorlton 2017-03-16 19:38 ` Alex Thorlton 2017-03-17 2:21 ` Aaron Lu 2017-03-17 2:21 ` Aaron Lu 2017-03-20 19:15 ` Alex Thorlton 2017-03-20 19:15 ` Alex Thorlton
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