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Message-ID: <xr93d3r79xux.fsf@ninji.mtv.corp.google.com>
Date: Mon, 18 Oct 2010 11:09:26 -0700
From: Greg Thelen <gthelen@...gle.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
containers@...ts.osdl.org, Andrea Righi <arighi@...eler.com>,
Balbir Singh <balbir@...ux.vnet.ibm.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH 00/10] memcg: per cgroup dirty page accounting
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:
> On Sun, 3 Oct 2010 23:57:55 -0700
> Greg Thelen <gthelen@...gle.com> wrote:
>
>> Greg Thelen (10):
>> memcg: add page_cgroup flags for dirty page tracking
>> memcg: document cgroup dirty memory interfaces
>> memcg: create extensible page stat update routines
>> memcg: disable local interrupts in lock_page_cgroup()
>> memcg: add dirty page accounting infrastructure
>> memcg: add kernel calls for memcg dirty page stats
>> memcg: add dirty limits to mem_cgroup
>> memcg: add cgroupfs interface to memcg dirty limits
>> writeback: make determine_dirtyable_memory() static.
>> memcg: check memcg dirty limits in page writeback
>
> Greg, this is a patch on your set.
>
> mmotm-1014
> - memcg-reduce-lock-hold-time-during-charge-moving.patch
> (I asked Andrew to drop this)
> + your 1,2,3,5,6,7,8,9,10 (dropped patch "4")
>
> I'm grad if you merge this to your set as replacement of "4".
> I'll prepare a performance improvement patch and post it if this dirty_limit
> patches goes to -mm.
Thanks for the patch. I will merge your patch (below) as a replacement
of memcg dirty limits patch #4 and repost the entire series.
> Thank you for your work.
>
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> Now, at supporing dirty limit, there is a deadlock problem in accounting.
>
> 1. If pages are being migrated from a memcg, then updates to that
> memcg page statistics are protected by grabbing a bit spin lock
> using lock_page_cgroup(). In recent changes of dirty page accounting
> is updating memcg page accounting (specifically: num writeback pages)
> from IRQ context (softirq). Avoid a deadlocking nested spin lock attempt
> by irq on the local processor when grabbing the page_cgroup.
>
> 2. lock for update_stat is used only for avoiding race with move_account().
> So, IRQ awareness of lock_page_cgroup() itself is not a problem. The problem
> is in update_stat() and move_account().
>
> Then, this reworks locking scheme of update_stat() and move_account() by
> adding new lock bit PCG_MOVE_LOCK, which is always taken under IRQ disable.
>
> Trade-off
> * using lock_page_cgroup() + disable IRQ has some impacts on performance
> and I think it's bad to disable IRQ when it's not necessary.
> * adding a new lock makes move_account() slow. Score is here.
>
> Peformance Impact: moving a 8G anon process.
>
> Before:
> real 0m0.792s
> user 0m0.000s
> sys 0m0.780s
>
> After:
> real 0m0.854s
> user 0m0.000s
> sys 0m0.842s
>
> This score is bad but planned patches for optimization can reduce
> this impact.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> include/linux/page_cgroup.h | 31 ++++++++++++++++++++++++++++---
> mm/memcontrol.c | 9 +++++++--
> 2 files changed, 35 insertions(+), 5 deletions(-)
>
> Index: dirty_limit_new/include/linux/page_cgroup.h
> ===================================================================
> --- dirty_limit_new.orig/include/linux/page_cgroup.h
> +++ dirty_limit_new/include/linux/page_cgroup.h
> @@ -35,15 +35,18 @@ struct page_cgroup *lookup_page_cgroup(s
>
> enum {
> /* flags for mem_cgroup */
> - PCG_LOCK, /* page cgroup is locked */
> + PCG_LOCK, /* Lock for pc->mem_cgroup and following bits. */
> PCG_CACHE, /* charged as cache */
> PCG_USED, /* this object is in use. */
> - PCG_ACCT_LRU, /* page has been accounted for */
> + PCG_MIGRATION, /* under page migration */
> + /* flags for mem_cgroup and file and I/O status */
> + PCG_MOVE_LOCK, /* For race between move_account v.s. following bits */
> PCG_FILE_MAPPED, /* page is accounted as "mapped" */
> PCG_FILE_DIRTY, /* page is dirty */
> PCG_FILE_WRITEBACK, /* page is under writeback */
> PCG_FILE_UNSTABLE_NFS, /* page is NFS unstable */
> - PCG_MIGRATION, /* under page migration */
> + /* No lock in page_cgroup */
> + PCG_ACCT_LRU, /* page has been accounted for (under lru_lock) */
> };
>
> #define TESTPCGFLAG(uname, lname) \
> @@ -119,6 +122,10 @@ static inline enum zone_type page_cgroup
>
> static inline void lock_page_cgroup(struct page_cgroup *pc)
> {
> + /*
> + * Don't take this lock in IRQ context.
> + * This lock is for pc->mem_cgroup, USED, CACHE, MIGRATION
> + */
> bit_spin_lock(PCG_LOCK, &pc->flags);
> }
>
> @@ -127,6 +134,24 @@ static inline void unlock_page_cgroup(st
> bit_spin_unlock(PCG_LOCK, &pc->flags);
> }
>
> +static inline void move_lock_page_cgroup(struct page_cgroup *pc,
> + unsigned long *flags)
> +{
> + /*
> + * We know updates to pc->flags of page cache's stats are from both of
> + * usual context or IRQ context. Disable IRQ to avoid deadlock.
> + */
> + local_irq_save(*flags);
> + bit_spin_lock(PCG_MOVE_LOCK, &pc->flags);
> +}
> +
> +static inline void move_unlock_page_cgroup(struct page_cgroup *pc,
> + unsigned long *flags)
> +{
> + bit_spin_unlock(PCG_MOVE_LOCK, &pc->flags);
> + local_irq_restore(*flags);
> +}
> +
> #else /* CONFIG_CGROUP_MEM_RES_CTLR */
> struct page_cgroup;
>
> Index: dirty_limit_new/mm/memcontrol.c
> ===================================================================
> --- dirty_limit_new.orig/mm/memcontrol.c
> +++ dirty_limit_new/mm/memcontrol.c
> @@ -1784,6 +1784,7 @@ void mem_cgroup_update_page_stat(struct
> struct mem_cgroup *mem;
> struct page_cgroup *pc = lookup_page_cgroup(page);
> bool need_unlock = false;
> + unsigned long uninitialized_var(flags);
>
> if (unlikely(!pc))
> return;
> @@ -1795,7 +1796,7 @@ void mem_cgroup_update_page_stat(struct
> /* pc->mem_cgroup is unstable ? */
> if (unlikely(mem_cgroup_stealed(mem))) {
> /* take a lock against to access pc->mem_cgroup */
> - lock_page_cgroup(pc);
> + move_lock_page_cgroup(pc, &flags);
> need_unlock = true;
> mem = pc->mem_cgroup;
> if (!mem || !PageCgroupUsed(pc))
> @@ -1856,7 +1857,7 @@ void mem_cgroup_update_page_stat(struct
>
> out:
> if (unlikely(need_unlock))
> - unlock_page_cgroup(pc);
> + move_unlock_page_cgroup(pc, &flags);
> rcu_read_unlock();
> return;
> }
> @@ -2426,9 +2427,13 @@ static int mem_cgroup_move_account(struc
> struct mem_cgroup *from, struct mem_cgroup *to, bool uncharge)
> {
> int ret = -EINVAL;
> + unsigned long flags;
> +
> lock_page_cgroup(pc);
> if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
> + move_lock_page_cgroup(pc, &flags);
> __mem_cgroup_move_account(pc, from, to, uncharge);
> + move_unlock_page_cgroup(pc, &flags);
> ret = 0;
> }
> unlock_page_cgroup(pc);
--
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