Re: [PATCH -mmotm 3/3] memcg: dirty pages instrumentation

[Andrea Righi]

On Tue, Mar 02, 2010 at 02:48:56PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-03-01 at 22:23 +0100, Andrea Righi wrote:
> > Apply the cgroup dirty pages accounting and limiting infrastructure to
> > the opportune kernel functions.
> > 
> > Signed-off-by: Andrea Righi <arighi@xxxxxxxxxxx>
> > ---
> 
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 5a0f8f3..d83f41c 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> > @@ -137,13 +137,14 @@ static struct prop_descriptor vm_dirties;
> >   */
> >  static int calc_period_shift(void)
> >  {
> > -	unsigned long dirty_total;
> > +	unsigned long dirty_total, dirty_bytes;
> >  
> > -	if (vm_dirty_bytes)
> > -		dirty_total = vm_dirty_bytes / PAGE_SIZE;
> > +	dirty_bytes = mem_cgroup_dirty_bytes();
> > +	if (dirty_bytes)
> 
> So you don't think 0 is a valid max dirty amount?

A value of 0 means "disabled". It's used to select between dirty_ratio
or dirty_bytes. It's the same for the gloabl vm_dirty_* parameters.

> 
> > +		dirty_total = dirty_bytes / PAGE_SIZE;
> >  	else
> > -		dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
> > -				100;
> > +		dirty_total = (mem_cgroup_dirty_ratio() *
> > +				determine_dirtyable_memory()) / 100;
> >  	return 2 + ilog2(dirty_total - 1);
> >  }
> >  
> > @@ -408,14 +409,16 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
> >   */
> >  unsigned long determine_dirtyable_memory(void)
> >  {
> > -	unsigned long x;
> > -
> > -	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> > +	unsigned long memory;
> > +	s64 memcg_memory;
> >  
> > +	memory = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
> >  	if (!vm_highmem_is_dirtyable)
> > -		x -= highmem_dirtyable_memory(x);
> > -
> > -	return x + 1;	/* Ensure that we never return 0 */
> > +		memory -= highmem_dirtyable_memory(memory);
> > +	memcg_memory = mem_cgroup_page_stat(MEMCG_NR_DIRTYABLE_PAGES);
> > +	if (memcg_memory < 0)
> 
> And here you somehow return negative?
> 
> > +		return memory + 1;
> > +	return min((unsigned long)memcg_memory, memory + 1);
> >  }
> >  
> >  void
> > @@ -423,26 +426,28 @@ get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
> >  		 unsigned long *pbdi_dirty, struct backing_dev_info *bdi)
> >  {
> >  	unsigned long background;
> > -	unsigned long dirty;
> > +	unsigned long dirty, dirty_bytes, dirty_background;
> >  	unsigned long available_memory = determine_dirtyable_memory();
> >  	struct task_struct *tsk;
> >  
> > -	if (vm_dirty_bytes)
> > -		dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
> > +	dirty_bytes = mem_cgroup_dirty_bytes();
> > +	if (dirty_bytes)
> 
> zero not valid again
> 
> > +		dirty = DIV_ROUND_UP(dirty_bytes, PAGE_SIZE);
> >  	else {
> >  		int dirty_ratio;
> >  
> > -		dirty_ratio = vm_dirty_ratio;
> > +		dirty_ratio = mem_cgroup_dirty_ratio();
> >  		if (dirty_ratio < 5)
> >  			dirty_ratio = 5;
> >  		dirty = (dirty_ratio * available_memory) / 100;
> >  	}
> >  
> > -	if (dirty_background_bytes)
> > -		background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);
> > +	dirty_background = mem_cgroup_dirty_background_bytes();
> > +	if (dirty_background)
> 
> idem
> 
> > +		background = DIV_ROUND_UP(dirty_background, PAGE_SIZE);
> >  	else
> > -		background = (dirty_background_ratio * available_memory) / 100;
> > -
> > +		background = (mem_cgroup_dirty_background_ratio() *
> > +					available_memory) / 100;
> >  	if (background >= dirty)
> >  		background = dirty / 2;
> >  	tsk = current;
> > @@ -508,9 +513,13 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  		get_dirty_limits(&background_thresh, &dirty_thresh,
> >  				&bdi_thresh, bdi);
> >  
> > -		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > +		nr_reclaimable = mem_cgroup_page_stat(MEMCG_NR_RECLAIM_PAGES);
> > +		nr_writeback = mem_cgroup_page_stat(MEMCG_NR_WRITEBACK);
> > +		if ((nr_reclaimable < 0) || (nr_writeback < 0)) {
> > +			nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> >  					global_page_state(NR_UNSTABLE_NFS);
> 
> ??? why would a page_state be negative.. I see you return -ENOMEM on !
> cgroup, but how can one specify no dirty limit with this compiled in?
> 
> > -		nr_writeback = global_page_state(NR_WRITEBACK);
> > +			nr_writeback = global_page_state(NR_WRITEBACK);
> > +		}
> >  
> >  		bdi_nr_reclaimable = bdi_stat(bdi, BDI_DIRTY);
> >  		if (bdi_cap_account_unstable(bdi)) {
> > @@ -611,10 +620,12 @@ static void balance_dirty_pages(struct address_space *mapping,
> >  	 * In normal mode, we start background writeout at the lower
> >  	 * background_thresh, to keep the amount of dirty memory low.
> >  	 */
> > +	nr_reclaimable = mem_cgroup_page_stat(MEMCG_NR_RECLAIM_PAGES);
> > +	if (nr_reclaimable < 0)
> > +		nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> > +				global_page_state(NR_UNSTABLE_NFS);
> 
> Again..
> 
> >  	if ((laptop_mode && pages_written) ||
> > -	    (!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
> > -			       + global_page_state(NR_UNSTABLE_NFS))
> > -					  > background_thresh)))
> > +	    (!laptop_mode && (nr_reclaimable > background_thresh)))
> >  		bdi_start_writeback(bdi, NULL, 0);
> >  }
> >  
> > @@ -678,6 +689,8 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> >  	unsigned long dirty_thresh;
> >  
> >          for ( ; ; ) {
> > +		unsigned long dirty;
> > +
> >  		get_dirty_limits(&background_thresh, &dirty_thresh, NULL, NULL);
> >  
> >                  /*
> > @@ -686,10 +699,14 @@ void throttle_vm_writeout(gfp_t gfp_mask)
> >                   */
> >                  dirty_thresh += dirty_thresh / 10;      /* wheeee... */
> >  
> > -                if (global_page_state(NR_UNSTABLE_NFS) +
> > -			global_page_state(NR_WRITEBACK) <= dirty_thresh)
> > -                        	break;
> > -                congestion_wait(BLK_RW_ASYNC, HZ/10);
> > +
> > +		dirty = mem_cgroup_page_stat(MEMCG_NR_DIRTY_WRITEBACK_PAGES);
> > +		if (dirty < 0)
> > +			dirty = global_page_state(NR_UNSTABLE_NFS) +
> > +				global_page_state(NR_WRITEBACK);
> 
> and again..
> 
> > +		if (dirty <= dirty_thresh)
> > +			break;
> > +		congestion_wait(BLK_RW_ASYNC, HZ/10);
> >  
> >  		/*
> >  		 * The caller might hold locks which can prevent IO completion
> 
> This is ugly and broken.. I thought you'd agreed to something like:
> 
>  if (mem_cgroup_has_dirty_limit(cgroup))
>    use mem_cgroup numbers
>  else
>    use global numbers

I agree mem_cgroup_has_dirty_limit() is nicer. But we must do that under
RCU, so something like:

	rcu_read_lock();
	if (mem_cgroup_has_dirty_limit())
		mem_cgroup_get_page_stat()
	else
		global_page_state()
	rcu_read_unlock();

That is bad when mem_cgroup_has_dirty_limit() always returns false
(e.g., when memory cgroups are disabled). So I fallback to the old
interface.

What do you think about:

	mem_cgroup_lock();
	if (mem_cgroup_has_dirty_limit())
		mem_cgroup_get_page_stat()
	else
		global_page_state()
	mem_cgroup_unlock();

Where mem_cgroup_read_lock/unlock() simply expand to nothing when
memory cgroups are disabled.

> 
> That allows for a 0 dirty limit (which should work and basically makes
> all io synchronous).

IMHO it is better to reserve 0 for the special value "disabled" like the
global settings. A synchronous IO can be also achieved using a dirty
limit of 1.

> 
> Also, I'd put each of those in a separate function, like:
> 
> unsigned long reclaimable_pages(cgroup)
> {
>   if (mem_cgroup_has_dirty_limit(cgroup))
>     return mem_cgroup_page_stat(MEMCG_NR_RECLAIM_PAGES);
>   
>   return global_page_state(NR_FILE_DIRTY) + global_page_state(NR_NFS_UNSTABLE);
> }

Agreed.

> 
> Which raises another question, you should probably rebase on top of
> Trond's patches, which removes BDI_RECLAIMABLE, suggesting you also
> loose MEMCG_NR_RECLAIM_PAGES in favour of the DIRTY+UNSTABLE split.

OK, will look at Trond's work.

Thanks,
-Andrea
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