From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
Ingo Molnar <mingo@kernel.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Linux-MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()
Date: Thu, 26 Sep 2013 19:50:16 +0200 [thread overview]
Message-ID: <20130926175016.GI3657@laptop.programming.kicks-ass.net> (raw)
In-Reply-To: <20130926165840.GA863@redhat.com>
On Thu, Sep 26, 2013 at 06:58:40PM +0200, Oleg Nesterov wrote:
> Peter,
>
> Sorry. Unlikely I will be able to read this patch today. So let me
> ask another potentially wrong question without any thinking.
>
> On 09/26, Peter Zijlstra wrote:
> >
> > +void __get_online_cpus(void)
> > +{
> > +again:
> > + /* See __srcu_read_lock() */
> > + __this_cpu_inc(__cpuhp_refcount);
> > + smp_mb(); /* A matches B, E */
> > + __this_cpu_inc(cpuhp_seq);
> > +
> > + if (unlikely(__cpuhp_state == readers_block)) {
>
> OK. Either we should see state = BLOCK or the writer should notice the
> change in __cpuhp_refcount/seq. (altough I'd like to recheck this
> cpuhp_seq logic ;)
>
> > + atomic_inc(&cpuhp_waitcount);
> > + __put_online_cpus();
>
> OK, this does wake(cpuhp_writer).
>
> > void cpu_hotplug_begin(void)
> > {
> > ...
> > + /*
> > + * Notify new readers to block; up until now, and thus throughout the
> > + * longish synchronize_sched() above, new readers could still come in.
> > + */
> > + __cpuhp_state = readers_block;
> > +
> > + smp_mb(); /* E matches A */
> > +
> > + /*
> > + * If they don't see our writer of readers_block to __cpuhp_state,
> > + * then we are guaranteed to see their __cpuhp_refcount increment, and
> > + * therefore will wait for them.
> > + */
> > +
> > + /* Wait for all now active readers to complete. */
> > + wait_event(cpuhp_writer, cpuhp_readers_active_check());
>
> But. doesn't this mean that we need __wait_event() here as well?
>
> Isn't it possible that the reader sees BLOCK but the writer does _not_
> see the change in __cpuhp_refcount/cpuhp_seq? Those mb's guarantee
> "either", not "both".
But if the readers does see BLOCK it will not be an active reader no
more; and thus the writer doesn't need to observe and wait for it.
> Don't we need to ensure that we can't check cpuhp_readers_active_check()
> after wake(cpuhp_writer) was already called by the reader and before we
> take the same lock?
I'm too tired to fully grasp what you're asking here; but given the
previous answer I think not.
next prev parent reply other threads:[~2013-09-26 17:50 UTC|newest]
Thread overview: 180+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-10 9:31 [PATCH 0/50] Basic scheduler support for automatic NUMA balancing V7 Mel Gorman
2013-09-10 9:31 ` [PATCH 01/50] sched: monolithic code dump of what is being pushed upstream Mel Gorman
2013-09-11 0:58 ` Joonsoo Kim
2013-09-11 3:11 ` Hillf Danton
2013-09-13 8:11 ` Mel Gorman
2013-09-10 9:31 ` [PATCH 02/50] mm: numa: Document automatic NUMA balancing sysctls Mel Gorman
2013-09-10 9:31 ` [PATCH 03/50] sched, numa: Comment fixlets Mel Gorman
2013-09-10 9:31 ` [PATCH 04/50] mm: numa: Do not account for a hinting fault if we raced Mel Gorman
2013-09-10 9:31 ` [PATCH 05/50] mm: Wait for THP migrations to complete during NUMA hinting faults Mel Gorman
2013-09-10 9:31 ` [PATCH 06/50] mm: Prevent parallel splits during THP migration Mel Gorman
2013-09-10 9:31 ` [PATCH 07/50] mm: Account for a THP NUMA hinting update as one PTE update Mel Gorman
2013-09-16 12:36 ` Peter Zijlstra
2013-09-16 13:39 ` Rik van Riel
2013-09-16 14:54 ` Peter Zijlstra
2013-09-16 16:11 ` Mel Gorman
2013-09-16 16:37 ` Peter Zijlstra
2013-09-10 9:31 ` [PATCH 08/50] mm: numa: Sanitize task_numa_fault() callsites Mel Gorman
2013-09-10 9:31 ` [PATCH 09/50] mm: numa: Do not migrate or account for hinting faults on the zero page Mel Gorman
2013-09-10 9:31 ` [PATCH 10/50] sched: numa: Mitigate chance that same task always updates PTEs Mel Gorman
2013-09-10 9:31 ` [PATCH 11/50] sched: numa: Continue PTE scanning even if migrate rate limited Mel Gorman
2013-09-10 9:31 ` [PATCH 12/50] Revert "mm: sched: numa: Delay PTE scanning until a task is scheduled on a new node" Mel Gorman
2013-09-10 9:31 ` [PATCH 13/50] sched: numa: Initialise numa_next_scan properly Mel Gorman
2013-09-10 9:31 ` [PATCH 14/50] sched: Set the scan rate proportional to the memory usage of the task being scanned Mel Gorman
2013-09-16 15:18 ` Peter Zijlstra
2013-09-16 15:40 ` Mel Gorman
2013-09-10 9:31 ` [PATCH 15/50] sched: numa: Correct adjustment of numa_scan_period Mel Gorman
2013-09-10 9:31 ` [PATCH 16/50] mm: Only flush TLBs if a transhuge PMD is modified for NUMA pte scanning Mel Gorman
2013-09-10 9:31 ` [PATCH 17/50] mm: Do not flush TLB during protection change if !pte_present && !migration_entry Mel Gorman
2013-09-16 16:35 ` Peter Zijlstra
2013-09-17 17:00 ` Mel Gorman
2013-09-10 9:31 ` [PATCH 18/50] sched: numa: Slow scan rate if no NUMA hinting faults are being recorded Mel Gorman
2013-09-10 9:31 ` [PATCH 19/50] sched: Track NUMA hinting faults on per-node basis Mel Gorman
2013-09-10 9:32 ` [PATCH 20/50] sched: Select a preferred node with the most numa hinting faults Mel Gorman
2013-09-10 9:32 ` [PATCH 21/50] sched: Update NUMA hinting faults once per scan Mel Gorman
2013-09-10 9:32 ` [PATCH 22/50] sched: Favour moving tasks towards the preferred node Mel Gorman
2013-09-10 9:32 ` [PATCH 23/50] sched: Resist moving tasks towards nodes with fewer hinting faults Mel Gorman
2013-09-10 9:32 ` [PATCH 24/50] sched: Reschedule task on preferred NUMA node once selected Mel Gorman
2013-09-10 9:32 ` [PATCH 25/50] sched: Add infrastructure for split shared/private accounting of NUMA hinting faults Mel Gorman
2013-09-10 9:32 ` [PATCH 26/50] sched: Check current->mm before allocating NUMA faults Mel Gorman
2013-09-10 9:32 ` [PATCH 27/50] mm: numa: Scan pages with elevated page_mapcount Mel Gorman
2013-09-12 2:10 ` Hillf Danton
2013-09-13 8:11 ` Mel Gorman
2013-09-10 9:32 ` [PATCH 28/50] sched: Remove check that skips small VMAs Mel Gorman
2013-09-10 9:32 ` [PATCH 29/50] sched: Set preferred NUMA node based on number of private faults Mel Gorman
2013-09-10 9:32 ` [PATCH 30/50] sched: Do not migrate memory immediately after switching node Mel Gorman
2013-09-10 9:32 ` [PATCH 31/50] sched: Avoid overloading CPUs on a preferred NUMA node Mel Gorman
2013-09-10 9:32 ` [PATCH 32/50] sched: Retry migration of tasks to CPU on a preferred node Mel Gorman
2013-09-10 9:32 ` [PATCH 33/50] sched: numa: increment numa_migrate_seq when task runs in correct location Mel Gorman
2013-09-10 9:32 ` [PATCH 34/50] sched: numa: Do not trap hinting faults for shared libraries Mel Gorman
[not found] ` <E81554BCB8813E49A8916AACC0503A851844C937@lc-shmail3.SHANGHAI.LEADCORETECH.COM>
2013-09-17 8:05 ` ????: " Mel Gorman
2013-09-10 9:32 ` [PATCH 35/50] mm: numa: Only trap pmd hinting faults if we would otherwise trap PTE faults Mel Gorman
2013-09-10 9:32 ` [PATCH 36/50] stop_machine: Introduce stop_two_cpus() Mel Gorman
2013-09-10 9:32 ` [PATCH 37/50] sched: Introduce migrate_swap() Mel Gorman
2013-09-17 14:30 ` [PATCH] hotplug: Optimize {get,put}_online_cpus() Peter Zijlstra
2013-09-17 16:20 ` Mel Gorman
2013-09-17 16:45 ` Peter Zijlstra
2013-09-18 15:49 ` Peter Zijlstra
2013-09-19 14:32 ` Peter Zijlstra
2013-09-21 16:34 ` Oleg Nesterov
2013-09-21 19:13 ` Oleg Nesterov
2013-09-23 9:29 ` Peter Zijlstra
2013-09-23 17:32 ` Oleg Nesterov
2013-09-24 20:24 ` Peter Zijlstra
2013-09-24 21:02 ` Peter Zijlstra
2013-09-25 15:55 ` Oleg Nesterov
2013-09-25 16:59 ` Paul E. McKenney
2013-09-25 17:43 ` Peter Zijlstra
2013-09-25 17:50 ` Oleg Nesterov
2013-09-25 18:40 ` Peter Zijlstra
2013-09-25 21:22 ` Paul E. McKenney
2013-09-26 11:10 ` Peter Zijlstra
[not found] ` <20130926155321.GA4342@redhat.com>
2013-09-26 16:13 ` Peter Zijlstra
2013-09-26 16:14 ` Oleg Nesterov
2013-09-26 16:40 ` Peter Zijlstra
2013-09-26 16:58 ` Oleg Nesterov
2013-09-26 17:50 ` Peter Zijlstra [this message]
2013-09-27 18:15 ` Oleg Nesterov
2013-09-27 20:41 ` Peter Zijlstra
2013-09-28 12:48 ` Oleg Nesterov
2013-09-28 14:47 ` Peter Zijlstra
2013-09-28 16:31 ` Oleg Nesterov
2013-09-30 20:11 ` Rafael J. Wysocki
2013-10-01 17:11 ` Srivatsa S. Bhat
2013-10-01 17:36 ` Peter Zijlstra
2013-10-01 17:45 ` Oleg Nesterov
2013-10-01 17:56 ` Peter Zijlstra
2013-10-01 18:07 ` Oleg Nesterov
2013-10-01 19:05 ` Paul E. McKenney
2013-10-02 12:16 ` Oleg Nesterov
2013-10-02 9:08 ` Peter Zijlstra
2013-10-02 12:13 ` Oleg Nesterov
2013-10-02 12:25 ` Peter Zijlstra
2013-10-02 13:31 ` Peter Zijlstra
2013-10-02 14:00 ` Oleg Nesterov
2013-10-02 15:17 ` Peter Zijlstra
2013-10-02 16:31 ` Oleg Nesterov
2013-10-02 17:52 ` Paul E. McKenney
2013-10-01 19:03 ` Srivatsa S. Bhat
2013-10-01 18:14 ` Srivatsa S. Bhat
2013-10-01 18:56 ` Srivatsa S. Bhat
2013-10-02 10:14 ` Srivatsa S. Bhat
2013-09-28 20:46 ` Paul E. McKenney
2013-10-01 3:56 ` Paul E. McKenney
2013-10-01 14:14 ` Oleg Nesterov
2013-10-01 14:45 ` Paul E. McKenney
2013-10-01 14:48 ` Peter Zijlstra
2013-10-01 15:24 ` Paul E. McKenney
2013-10-01 15:34 ` Oleg Nesterov
2013-10-01 15:00 ` Oleg Nesterov
2013-09-29 13:56 ` Oleg Nesterov
2013-10-01 15:38 ` Paul E. McKenney
2013-10-01 15:40 ` Oleg Nesterov
2013-10-01 20:40 ` Paul E. McKenney
2013-09-23 14:50 ` Steven Rostedt
2013-09-23 14:54 ` Peter Zijlstra
2013-09-23 15:13 ` Steven Rostedt
2013-09-23 15:22 ` Peter Zijlstra
2013-09-23 15:59 ` Steven Rostedt
2013-09-23 16:02 ` Peter Zijlstra
2013-09-23 15:50 ` Paul E. McKenney
2013-09-23 16:01 ` Peter Zijlstra
2013-09-23 17:04 ` Paul E. McKenney
2013-09-23 17:30 ` Peter Zijlstra
2013-09-23 17:50 ` Oleg Nesterov
2013-09-24 12:38 ` Peter Zijlstra
2013-09-24 14:42 ` Paul E. McKenney
2013-09-24 16:09 ` Peter Zijlstra
2013-09-24 16:31 ` Oleg Nesterov
2013-09-24 21:09 ` Paul E. McKenney
2013-09-24 16:03 ` Oleg Nesterov
2013-09-24 16:43 ` Steven Rostedt
2013-09-24 17:06 ` Oleg Nesterov
2013-09-24 17:47 ` Paul E. McKenney
2013-09-24 18:00 ` Oleg Nesterov
2013-09-24 20:35 ` Peter Zijlstra
2013-09-25 15:16 ` Oleg Nesterov
2013-09-25 15:35 ` Peter Zijlstra
2013-09-25 16:33 ` Oleg Nesterov
2013-09-24 16:49 ` Paul E. McKenney
2013-09-24 16:54 ` Peter Zijlstra
2013-09-24 17:02 ` Oleg Nesterov
2013-09-24 16:51 ` Peter Zijlstra
2013-09-24 16:39 ` Steven Rostedt
2013-09-29 18:36 ` [RFC] introduce synchronize_sched_{enter,exit}() Oleg Nesterov
2013-09-29 20:01 ` Paul E. McKenney
2013-09-30 12:42 ` Oleg Nesterov
2013-09-29 21:34 ` Steven Rostedt
2013-09-30 13:03 ` Oleg Nesterov
2013-09-30 12:59 ` Peter Zijlstra
2013-09-30 14:24 ` Peter Zijlstra
2013-09-30 15:06 ` Peter Zijlstra
2013-09-30 16:58 ` Oleg Nesterov
2013-09-30 16:38 ` Oleg Nesterov
2013-10-02 14:41 ` Peter Zijlstra
2013-10-03 7:04 ` Ingo Molnar
2013-10-03 7:43 ` Peter Zijlstra
2013-09-17 14:32 ` [PATCH 37/50] sched: Introduce migrate_swap() Peter Zijlstra
2013-09-10 9:32 ` [PATCH 38/50] sched: numa: Use a system-wide search to find swap/migration candidates Mel Gorman
2013-09-10 9:32 ` [PATCH 39/50] sched: numa: Favor placing a task on the preferred node Mel Gorman
2013-09-10 9:32 ` [PATCH 40/50] mm: numa: Change page last {nid,pid} into {cpu,pid} Mel Gorman
2013-09-10 9:32 ` [PATCH 41/50] sched: numa: Use {cpu, pid} to create task groups for shared faults Mel Gorman
2013-09-12 12:42 ` Hillf Danton
2013-09-12 14:40 ` Mel Gorman
2013-09-12 12:45 ` Hillf Danton
2013-09-10 9:32 ` [PATCH 42/50] sched: numa: Report a NUMA task group ID Mel Gorman
2013-09-10 9:32 ` [PATCH 43/50] mm: numa: Do not group on RO pages Mel Gorman
2013-09-10 9:32 ` [PATCH 44/50] sched: numa: stay on the same node if CLONE_VM Mel Gorman
2013-09-10 9:32 ` [PATCH 45/50] sched: numa: use group fault statistics in numa placement Mel Gorman
2013-09-10 9:32 ` [PATCH 46/50] sched: numa: Prevent parallel updates to group stats during placement Mel Gorman
2013-09-20 9:55 ` Peter Zijlstra
2013-09-20 12:31 ` Mel Gorman
2013-09-20 12:36 ` Peter Zijlstra
2013-09-20 13:31 ` Mel Gorman
2013-09-10 9:32 ` [PATCH 47/50] sched: numa: add debugging Mel Gorman
2013-09-10 9:32 ` [PATCH 48/50] sched: numa: Decide whether to favour task or group weights based on swap candidate relationships Mel Gorman
2013-09-10 9:32 ` [PATCH 49/50] sched: numa: fix task or group comparison Mel Gorman
2013-09-10 9:32 ` [PATCH 50/50] sched: numa: Avoid migrating tasks that are placed on their preferred node Mel Gorman
2013-09-11 2:03 ` [PATCH 0/50] Basic scheduler support for automatic NUMA balancing V7 Rik van Riel
2013-09-14 2:57 ` Bob Liu
2013-09-30 10:30 ` Mel Gorman
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