linux-kernel - Re: [RFC][PATCH 7/7][memcg] use spin lock instead of bit_spin_lock in page

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Date:	Tue, 27 Jul 2010 23:16:54 -0700
From:	Greg Thelen <gthelen@...gle.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-mm\@kvack.org" <linux-mm@...ck.org>,
	"nishimura\@mxp.nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir\@linux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	m-ikeda@...jp.nec.com,
	"akpm\@linux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 7/7][memcg] use spin lock instead of bit_spin_lock in page_cgroup

KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> writes:

> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
>
> This patch replaces page_cgroup's bit_spinlock with spinlock. In general,
> spinlock has good implementation than bit_spin_lock and we should use
> it if we have a room for it. In 64bit arch, we have extra 4bytes.
> Let's use it.
>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> --
> Index: mmotm-0719/include/linux/page_cgroup.h
> ===================================================================
> --- mmotm-0719.orig/include/linux/page_cgroup.h
> +++ mmotm-0719/include/linux/page_cgroup.h
> @@ -10,8 +10,14 @@
>   * All page cgroups are allocated at boot or memory hotplug event,
>   * then the page cgroup for pfn always exists.
>   */
> +#ifdef CONFIG_64BIT
> +#define PCG_HAS_SPINLOCK
> +#endif
>  struct page_cgroup {
>  	unsigned long flags;
> +#ifdef PCG_HAS_SPINLOCK
> +	spinlock_t	lock;
> +#endif
>  	unsigned short mem_cgroup;	/* ID of assigned memory cgroup */
>  	unsigned short blk_cgroup;	/* Not Used..but will be. */
>  	struct page *page;
> @@ -90,6 +96,16 @@ static inline enum zone_type page_cgroup
>  	return page_zonenum(pc->page);
>  }
>  
> +#ifdef PCG_HAS_SPINLOCK
> +static inline void lock_page_cgroup(struct page_cgroup *pc)
> +{
> +	spin_lock(&pc->lock);
> +}

This is minor issue, but this patch breaks usage of PageCgroupLocked().
Example from __mem_cgroup_move_account() cases panic:
	VM_BUG_ON(!PageCgroupLocked(pc));

I assume that this patch should also delete the following:
- PCG_LOCK definition from page_cgroup.h
- TESTPCGFLAG(Locked, LOCK) from page_cgroup.h
- PCGF_LOCK from memcontrol.c

> +static inline void unlock_page_cgroup(struct page_cgroup *pc)
> +{
> +	spin_unlock(&pc->lock);
> +}
> +#else
>  static inline void lock_page_cgroup(struct page_cgroup *pc)
>  {
>  	bit_spin_lock(PCG_LOCK, &pc->flags);
> @@ -99,6 +115,7 @@ static inline void unlock_page_cgroup(st
>  {
>  	bit_spin_unlock(PCG_LOCK, &pc->flags);
>  }
> +#endif
>  
>  static inline void SetPCGFileFlag(struct page_cgroup *pc, int idx)
>  {
> Index: mmotm-0719/mm/page_cgroup.c
> ===================================================================
> --- mmotm-0719.orig/mm/page_cgroup.c
> +++ mmotm-0719/mm/page_cgroup.c
> @@ -17,6 +17,9 @@ __init_page_cgroup(struct page_cgroup *p
>  	pc->mem_cgroup = 0;
>  	pc->page = pfn_to_page(pfn);
>  	INIT_LIST_HEAD(&pc->lru);
> +#ifdef PCG_HAS_SPINLOCK
> +	spin_lock_init(&pc->lock);
> +#endif
>  }
>  static unsigned long total_usage;
>  
--
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