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Date: Mon, 13 Sep 2010 22:56:19 +0530
From: Balbir Singh <balbir@...ux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
gthelen@...gle.com,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] memcg: avoid lock in updating file_mapped (Was fix race
in file_mapped accouting flag management
* KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-09-13 17:01:51]:
>
> Very sorry, subject was wrong..(reposting).
>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> At accounting file events per memory cgroup, we need to find memory cgroup
> via page_cgroup->mem_cgroup. Now, we use lock_page_cgroup() for guarantee
> pc->mem_cgroup is not overwritten while we make use of it.
>
> But, considering the context which page-cgroup for files are accessed,
> we can use alternative light-weight mutual execusion in the most case.
>
> At handling file-caches, the only race we have to take care of is "moving"
> account, IOW, overwriting page_cgroup->mem_cgroup.
> (See comment in the patch)
>
> Unlike charge/uncharge, "move" happens not so frequently. It happens only when
> rmdir() and task-moving (with a special settings.)
> This patch adds a race-checker for file-cache-status accounting v.s. account
> moving. The new per-cpu-per-memcg counter MEM_CGROUP_ON_MOVE is added.
> The routine for account move
> 1. Increment it before start moving
> 2. Call synchronize_rcu()
> 3. Decrement it after the end of moving.
> By this, file-status-counting routine can check it needs to call
> lock_page_cgroup(). In most case, I doesn't need to call it.
>
> Following is a perf data of a process which mmap()/munmap 32MB of file cache
> in a minute.
>
> Before patch:
> 28.25% mmap mmap [.] main
> 22.64% mmap [kernel.kallsyms] [k] page_fault
> 9.96% mmap [kernel.kallsyms] [k] mem_cgroup_update_file_mapped
> 3.67% mmap [kernel.kallsyms] [k] filemap_fault
> 3.50% mmap [kernel.kallsyms] [k] unmap_vmas
> 2.99% mmap [kernel.kallsyms] [k] __do_fault
> 2.76% mmap [kernel.kallsyms] [k] find_get_page
>
> After patch:
> 30.00% mmap mmap [.] main
> 23.78% mmap [kernel.kallsyms] [k] page_fault
> 5.52% mmap [kernel.kallsyms] [k] mem_cgroup_update_file_mapped
> 3.81% mmap [kernel.kallsyms] [k] unmap_vmas
> 3.26% mmap [kernel.kallsyms] [k] find_get_page
> 3.18% mmap [kernel.kallsyms] [k] __do_fault
> 3.03% mmap [kernel.kallsyms] [k] filemap_fault
> 2.40% mmap [kernel.kallsyms] [k] handle_mm_fault
> 2.40% mmap [kernel.kallsyms] [k] do_page_fault
>
> This patch reduces memcg's cost to some extent.
> (mem_cgroup_update_file_mapped is called by both of map/unmap)
>
> Note: It seems some more improvements are required..but no idea.
> maybe removing set/unset flag is required.
>
> Changelog: 20100913
> - decoupled with ID patches.
> - updated comments.
>
> Changelog: 20100901
> - changes id_to_memcg(pc, true) to be id_to_memcg(pc, false)
> in update_file_mapped()
> - updated comments on lock rule of update_file_mapped()
> Changelog: 20100825
> - added a comment about mc.lock
> - fixed bad lock.
> Changelog: 20100804
> - added a comment for possible optimization hint.
> Changelog: 20100730
> - some cleanup.
> Changelog: 20100729
> - replaced __this_cpu_xxx() with this_cpu_xxx
> (because we don't call spinlock)
> - added VM_BUG_ON().
>
> Reviewed-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> mm/memcontrol.c | 99 ++++++++++++++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 85 insertions(+), 14 deletions(-)
>
> Index: lockless-update/mm/memcontrol.c
> ===================================================================
> --- lockless-update.orig/mm/memcontrol.c
> +++ lockless-update/mm/memcontrol.c
> @@ -90,6 +90,7 @@ enum mem_cgroup_stat_index {
> MEM_CGROUP_STAT_PGPGOUT_COUNT, /* # of pages paged out */
> MEM_CGROUP_STAT_SWAPOUT, /* # of pages, swapped out */
> MEM_CGROUP_EVENTS, /* incremented at every pagein/pageout */
> + MEM_CGROUP_ON_MOVE, /* someone is moving account between groups */
>
> MEM_CGROUP_STAT_NSTATS,
> };
> @@ -1051,7 +1052,46 @@ static unsigned int get_swappiness(struc
> return swappiness;
> }
>
> -/* A routine for testing mem is not under move_account */
> +static void mem_cgroup_start_move(struct mem_cgroup *mem)
> +{
> + int cpu;
> + /* Because this is for moving account, reuse mc.lock */
> + spin_lock(&mc.lock);
> + for_each_possible_cpu(cpu)
for_each_possible_cpu() might be too much, no?
I recommend we use a get_online_cpus()/put_online_cpus() pair
around the call and optimize.
> + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) += 1;
> + spin_unlock(&mc.lock);
> +
> + synchronize_rcu();
> +}
> +
> +static void mem_cgroup_end_move(struct mem_cgroup *mem)
> +{
> + int cpu;
> +
> + if (!mem)
> + return;
> + spin_lock(&mc.lock);
> + for_each_possible_cpu(cpu)
> + per_cpu(mem->stat->count[MEM_CGROUP_ON_MOVE], cpu) -= 1;
Same as above
--
Three Cheers,
Balbir
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