Re: Re: [Xen-devel] [PATCH v7 2/3] xen/blkback: Squeeze page pools if a memory pressure is detected

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Dec 12, 2019 at 05:06:58PM +0100, SeongJae Park wrote:
> On Thu, 12 Dec 2019 16:27:57 +0100 "Roger Pau Monné" <roger.pau@citrix.com> wrote:
> 
> > > diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> > > index fd1e19f1a49f..98823d150905 100644
> > > --- a/drivers/block/xen-blkback/blkback.c
> > > +++ b/drivers/block/xen-blkback/blkback.c
> > > @@ -142,6 +142,21 @@ static inline bool persistent_gnt_timeout(struct persistent_gnt *persistent_gnt)
> > >  		HZ * xen_blkif_pgrant_timeout);
> > >  }
> > >  
> > > +/* Once a memory pressure is detected, squeeze free page pools for a while. */
> > > +static unsigned int buffer_squeeze_duration_ms = 10;
> > > +module_param_named(buffer_squeeze_duration_ms,
> > > +		buffer_squeeze_duration_ms, int, 0644);
> > > +MODULE_PARM_DESC(buffer_squeeze_duration_ms,
> > > +"Duration in ms to squeeze pages buffer when a memory pressure is detected");
> > > +
> > > +static unsigned long buffer_squeeze_end;
> > > +
> > > +void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
> > > +{
> > > +	buffer_squeeze_end = jiffies +
> > > +		msecs_to_jiffies(buffer_squeeze_duration_ms);
> > 
> > I'm not sure this is fully correct. This function will be called for
> > each blkback instance, but the timeout is stored in a global variable
> > that's shared between all blkback instances. Shouldn't this timeout be
> > stored in xen_blkif so each instance has it's own local variable?
> > 
> > Or else in the case you have 1k blkback instances the timeout is
> > certainly going to be longer than expected, because each call to
> > xen_blkbk_reclaim_memory will move it forward.
> 
> Agreed that.  I think the extended timeout would not make a visible
> performance, though, because the time that 1k-loop take would be short enough
> to be ignored compared to the millisecond-scope duration.
> 
> I took this way because I wanted to minimize such structural changes as far as
> I can, as this is just a point-fix rather than ultimate solution.  That said,
> it is not fully correct and very confusing.  My another colleague also pointed
> out it in internal review.  Correct solution would be to adding a variable in
> the struct as you suggested or avoiding duplicated update of the variable by
> initializing the variable once the squeezing duration passes.  I would prefer
> the later way, as it is more straightforward and still not introducing
> structural change.  For example, it might be like below:
> 
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index f41c698dd854..6856c8ef88de 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -152,8 +152,9 @@ static unsigned long buffer_squeeze_end;
>  
>  void xen_blkbk_reclaim_memory(struct xenbus_device *dev)
>  {
> -       buffer_squeeze_end = jiffies +
> -               msecs_to_jiffies(buffer_squeeze_duration_ms);
> +       if (!buffer_squeeze_end)
> +               buffer_squeeze_end = jiffies +
> +                       msecs_to_jiffies(buffer_squeeze_duration_ms);
>  }
>  
>  static inline int get_free_page(struct xen_blkif_ring *ring, struct page **page)
> @@ -669,10 +670,13 @@ int xen_blkif_schedule(void *arg)
>                 }
>  
>                 /* Shrink the free pages pool if it is too large. */
> -               if (time_before(jiffies, buffer_squeeze_end))
> +               if (time_before(jiffies, buffer_squeeze_end)) {
>                         shrink_free_pagepool(ring, 0);
> -               else
> +               } else {
> +                       if (unlikely(buffer_squeeze_end))
> +                               buffer_squeeze_end = 0;
>                         shrink_free_pagepool(ring, max_buffer_pages);
> +               }
>  
>                 if (log_stats && time_after(jiffies, ring->st_print))
>                         print_stats(ring);
> 
> May I ask you what way would you prefer?

I'm not particularly found of this approach, as I think it's racy. Ie:
you would have to add some kind of lock to make sure the contents of
buffer_squeeze_end stay unmodified during the read and set cycle, or
else xen_blkif_schedule will race with xen_blkbk_reclaim_memory.

This is likely not a big deal ATM since the code will work as
expected in most cases AFAICT, but I would still prefer to have a
per-instance buffer_squeeze_end added to xen_blkif, given that the
callback is per-instance. I wouldn't call it a structural change, it's
just adding a variable to a struct instead of having a shared one, but
the code is almost the same as the current version.

Thanks, Roger.