Re: [PATCH RFC] SUNRPC: Refresh rq_pages using a bulk page allocator

[Mel Gorman <mgorman@techsingularity.net>]

On Mon, Feb 22, 2021 at 02:58:04PM +0000, Chuck Lever wrote:
> > There is a conflict at the end where rq_page_end gets updated. The 5.11
> > code assumes that the loop around the allocator definitely gets all
> > the required pages. What tree is this patch based on and is it going in
> > during this merge window? While the conflict is "trivial" to resolve,
> > it would be buggy because on retry, "i" will be pointing to the wrong
> > index and pages potentially leak. Rather than guessing, I'd prefer to
> > base a series on code you've tested.
> 
> I posted this patch as a proof of concept. There is a clean-up patch
> that goes before it to deal properly with rq_page_end. I can post
> both if you really want to apply this and play with it.
> 

It's for the best. It doesn't belong in the series as such but it may
affect what the bulk allocator usage looks like.

> 
> > The slowpath for the bulk allocator also sucks a bit for the semantics
> > required by this caller. As the bulk allocator does not walk the zonelist,
> > it can return failures prematurely -- fine for an optimistic bulk allocator
> > that can return a subset of pages but not for this caller which really
> > wants those pages. The allocator may need NOFAIL-like semantics to walk
> > the zonelist if the caller really requires success or at least walk the
> > zonelist if the preferred zone is low on pages. This patch would also
> > need to preserve the schedule_timeout behaviour so it does not use a lot
> > of CPU time retrying allocations in the presense of memory pressure.
> 
> Waiting half a second before trying again seems like overkill, though.
> 

It is both overkill and time is not directly correlated with memory
pressure. However, I would also suggest removing the timeout as a separate
patch as it's not related to the bulk allocator in case someone does
encounter a high CPU usage problem and bisects it the patch using the
bulk allocator for the first time.

-- 
Mel Gorman
SUSE Labs