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@xxxxxxxxx>
> > > ---
> > > 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@xxxxxxxxx>
> >
> >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
