Re: [PATCH v3] rcu/kvfree: Prevents cache growing when the backoff_page_cache_fill is set

[Paul E. McKenney]

On Tue, Apr 11, 2023 at 04:58:22PM +0200, Uladzislau Rezki wrote:
> On Tue, Apr 11, 2023 at 02:42:27PM +0000, Zhang, Qiang1 wrote:
> > > > Currently, in kfree_rcu_shrink_scan(), the drain_page_cache() is
> > > > executed before kfree_rcu_monitor() to drain page cache, if the bnode
> > > > structure's->gp_snap has done, the kvfree_rcu_bulk() will fill the
> > > > page cache again in kfree_rcu_monitor(), this commit add a check
> > > > for krcp structure's->backoff_page_cache_fill in put_cached_bnode(),
> > > > if the krcp structure's->backoff_page_cache_fill is set, prevent page
> > > > cache growing and disable allocated page in fill_page_cache_func().
> > > > 
> > > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> > > >
> > > >Much improved!  But still some questions below...
> > > >
> > > >							Thanx, Paul
> > > >
> > > > ---
> > > >  kernel/rcu/tree.c | 4 +++-
> > > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > > index cc34d13be181..9d9d3772cc45 100644
> > > > --- a/kernel/rcu/tree.c
> > > > +++ b/kernel/rcu/tree.c
> > > > @@ -2908,6 +2908,8 @@ static inline bool
> > > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > > >  	struct kvfree_rcu_bulk_data *bnode)
> > > >  {
> > > > +	if (atomic_read(&krcp->backoff_page_cache_fill))
> > > > +		return false;
> > > >
> > > >This will mean that under low-memory conditions, we will keep zero
> > > >pages in ->bkvcache.  All attempts to put something there will fail.
> > > >
> > > >This is probably not an issue for structures containing an rcu_head
> > > >that are passed to kfree_rcu(p, field), but doesn't this mean that
> > > >kfree_rcu_mightsleep() unconditionally invokes synchronize_rcu()?
> > > >This could seriously slow up freeing under low-memory conditions,
> > > >which might exacerbate the low-memory conditions.
> > > 
> > > Thanks for mentioning this, I didn't think of this before😊.
> > > 
> > > >
> > > >Is this really what we want?  Zero cached rather than just fewer cached?
> > > >
> > > >
> > > >
> > > >  	// Check the limit.
> > > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > > >  		return false;
> > > > @@ -3221,7 +3223,7 @@ static void fill_page_cache_func(struct work_struct *work)
> > > >  	int i;
> > > >  
> > > >  	nr_pages = atomic_read(&krcp->backoff_page_cache_fill) ?
> > > > -		1 : rcu_min_cached_objs;
> > > > +		0 : rcu_min_cached_objs;
> > > >  
> > > >  	for (i = 0; i < nr_pages; i++) {
> > > >
> > > >I am still confused as to why we start "i" at zero rather than at
> > > >->nr_bkv_objs.  What am I missing here?
> > > 
> > > 
> > > No, you are right, I missed this place. 
> > > 
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2908,6 +2908,8 @@ static inline bool
> > >  put_cached_bnode(struct kfree_rcu_cpu *krcp,
> > >         struct kvfree_rcu_bulk_data *bnode)
> > >  {
> > > +       if (atomic_read(&krcp->backoff_page_cache_fill))
> > > +               return false;
> > >
> > >This is broken, unfortunately. If a low memory condition we fill
> > >fill a cache with at least one page anyway because of we do not want
> > >to hit a slow path.
> > 
> > Thanks remind, please ignore my v4 patch,   how about the following?
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 41daae3239b5..e2e8412e687f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3238,6 +3238,9 @@ static void fill_page_cache_func(struct work_struct *work)
> >                         free_page((unsigned long) bnode);
> >                         break;
> >                 }
> > +
> > +               if (atomic_read(&krcp->backoff_page_cache_fill))
> > +                       break;
> >         }
> It does not fix an "issue" you are reporting. kvfree_rcu_bulk() function
> can still fill it back. IMHO, the solution here is to disable cache if
> a low memory condition and enable back later on.
> 
> The cache size is controlled by the rcu_min_cached_objs variable. We can
> set it to 1 and restore it back to original value to make the cache operating
> as before.

It would be best to use a second variable for this.  Users might get
annoyed if their changes to rcu_min_cached_objs got overwritten once
things got set back to normal operation.

							Thanx, Paul