Re: [RFC][PATCH 7/7][memcg] use spin lock instead of bit_spin_lock in page_cgroup

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* Greg Thelen <gthelen@google.com> [2010-07-27 23:16:54]:

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


Good catch! But from my understanding of the code we use spinlock_t
only for 64 bit systems, so we still need the PCG* and TESTPGFLAGS.
 
> > +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;
> >  

-- 
	Three Cheers,
	Balbir

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