Re: [PATCH] mm: don't call lru draining in the nested lru_cache_disable

[Minchan Kim]

On Mon, Dec 06, 2021 at 03:34:19PM -0800, Minchan Kim wrote:
> On Mon, Dec 06, 2021 at 03:04:21PM -0800, Andrew Morton wrote:
> > On Mon,  6 Dec 2021 14:10:06 -0800 Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > 
> > > lru_cache_disable involves IPIs to drain pagevec of each core,
> > > which sometimes takes quite long time to complete depending
> > > on cpu's business, which makes allocation too slow up to
> > > sveral hundredth milliseconds. Furthermore, the repeated draining
> > > in the alloc_contig_range makes thing worse considering caller
> > > of alloc_contig_range usually tries multiple times in the loop.
> > > 
> > > This patch makes the lru_cache_disable aware of the fact the
> > > pagevec was already disabled. With that, user of alloc_contig_range
> > > can disable the lru cache in advance in their context during the
> > > repeated trial so they can avoid the multiple costly draining
> > > in cma allocation.
> > 
> > Isn't this racy?
> >  
> > > ...
> > >
> > > @@ -859,7 +869,12 @@ atomic_t lru_disable_count = ATOMIC_INIT(0);
> > >   */
> > >  void lru_cache_disable(void)
> > >  {
> > > -	atomic_inc(&lru_disable_count);
> > > +	/*
> > > +	 * If someone is already disabled lru_cache, just return with
> > > +	 * increasing the lru_disable_count.
> > > +	 */
> > > +	if (atomic_inc_not_zero(&lru_disable_count))
> > > +		return;
> > >  #ifdef CONFIG_SMP
> > >  	/*
> > >  	 * lru_add_drain_all in the force mode will schedule draining on
> > > @@ -873,6 +888,7 @@ void lru_cache_disable(void)
> > >  #else
> > >  	lru_add_and_bh_lrus_drain();
> > >  #endif
> > 
> > There's a window here where lru_disable_count==0 and new pages can get
> > added to lru?
> 
> Indeed. If __lru_add_drain_all in core A didn't run yet but increased
> the disable count already, lru_cache_disable in core B will not see
> those pages in the LRU. Need to be fixed it.
> 
> Thanks, Andrew.