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Date:   Thu, 6 Apr 2023 17:04:03 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     "Zhang, Qiang1" <qiang1.zhang@intel.com>
Cc:     Uladzislau Rezki <urezki@gmail.com>,
        "frederic@kernel.org" <frederic@kernel.org>,
        "joel@joelfernandes.org" <joel@joelfernandes.org>,
        "qiang.zhang1211@gmail.com" <qiang.zhang1211@gmail.com>,
        "rcu@vger.kernel.org" <rcu@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] rcu/kvfree: Prevents cache growing when the
 backoff_page_cache_fill is set
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On Thu, Apr 06, 2023 at 11:11:37PM +0000, Zhang, Qiang1 wrote:
> >>On Thu, Apr 06, 2023 at 06:37:53AM +0200, Uladzislau Rezki wrote:
> > On Thu, Apr 06, 2023 at 08:12:38AM +0800, Zqiang 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.
> > > 
> > > Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> > > ---
> > >  kernel/rcu/tree.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 9cc0a7766fd2..f25430ae1936 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2907,6 +2907,8 @@ 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;
> > >  	// Check the limit.
> > >  	if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
> > >  		return false;
> > > -- 
> > > 2.32.0
> > >
> > Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> >
> >Thank you both!
> >
> >One question, though.  Might it be better to instead modify the "for"
> >loop in fill_page_cache_func() to start at krcp->nr_bkv_objs instead
> >of starting at zero?  That way, we still provide a single page under
> >low-memory conditions, but provide rcu_min_cached_objs of them if memory
> >is plentiful.
> >
> >Alternatively, if we really don't want to allow any pages at all under
> >low-memory conditions, shouldn't the fill_page_cache_func() set nr_pages
> >to zero (instead of the current 1) when the krcp->backoff_page_cache_fill
> >flag is set?  
> 
> Hi, Paul
> 
> If the krcp->backoff_page_cache_fill is true, the put_cached_bnode () return false,
> the allocated single page will also be freed in fill_page_cache_func().
> 
> or it would be better not to allocate under memory pressure.

That was my thought.  ;-)

> How about like this?
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9cc0a7766fd2..94aedbc3da36 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2907,6 +2907,8 @@ 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;
>         // Check the limit.
>         if (krcp->nr_bkv_objs >= rcu_min_cached_objs)
>                 return false;
> @@ -3220,7 +3222,7 @@ 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++) {

The other question is why this loop does not allow for any pages
that might already be allocated, thus perhaps looking like this:

		for (i = krcp->nr_bkv_objs; i < nr_pages; i++) {

Or do we somehow know that krcp->nr_bkv_objs is equal to zero?  (I am not
seeing this, but I do feel the need to ask.)

							Thanx, Paul

>                 bnode = (struct kvfree_rcu_bulk_data *)
> 
> Thanks
> Zqiang
> 
> >This would likely mean also breaking out of that loop if
> >krcp->backoff_page_cache_fill was set in the meantime (which happens
> >implicitly with Zqiang's patch).
> >
> >Or am I missing something subtle here?
> >
> >							Thanx, Paul