Re: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.

[Joel Fernandes]

On Wed, Sep 30, 2020 at 04:39:53PM +0200, Vlastimil Babka wrote:
> On 9/30/20 12:07 AM, Uladzislau Rezki wrote:
> > On Tue, Sep 29, 2020 at 12:15:34PM +0200, Vlastimil Babka wrote:
> >> On 9/18/20 9:48 PM, Uladzislau Rezki (Sony) wrote:
> >> 
> >> After reading all the threads and mulling over this, I am going to deflect from
> >> Mel and Michal and not oppose the idea of lockless allocation. I would even
> >> prefer to do it via the gfp flag and not a completely separate path. Not using
> >> the exact code from v1, I think it could be done in a way that we don't actually
> >> look at the new flag until we find that pcplist is empty - which should not
> >> introduce overhead to the fast-fast path when pcpclist is not empty. It's more
> >> maintainable that adding new entry points, IMHO.
> >> 
> > Thanks for reading all that from the beginning! It must be tough due to the
> > fact there were lot of messages in the threads, so at least i was lost.
> > 
> > I agree that adding a new entry or separate lock-less function can be considered
> > as something that is hard to maintain. I have a question here. I mean about your
> > different look at it:
> > 
> > <snip>
> > bool is_pcp_cache_empty(gfp_t gfp)
> > {
> >     struct per_cpu_pages *pcp;
> >     struct zoneref *ref;
> >     unsigned long flags;
> >     bool empty;
> > 
> >     ref = first_zones_zonelist(node_zonelist(
> >             numa_node_id(), gfp), gfp_zone(gfp), NULL);
> >     if (!ref->zone)
> >             return true;
> > 
> >     local_irq_save(flags);
> >     pcp = &this_cpu_ptr(ref->zone->pageset)->pcp;
> >     empty = list_empty(&pcp->lists[gfp_migratetype(gfp)]);
> >     local_irq_restore(flags);
> > 
> >     return empty;
> > }
> > 
> > disable_irq();
> > if (!is_pcp_cache_empty(GFP_NOWAIT))
> >     __get_free_page(GFP_NOWAIT);
> > enable_irq();
> > <snip>
> > 
> > Do you mean to have something like above? I mean some extra API
> > function that returns true or false if fast-fast allocation can
> > either occur or not. Above code works just fine and never touches
> > main zone->lock.
> > 
> > i.e. Instead of introducing an extra GFP_LOCKLESS flag or any new
> > extra lock-less function. We could have something that checks a
> > pcp page cache list, thus it can guarantee that a request would
> > be accomplish using fast-fast path.
> 
> No, I meant going back to idea of new gfp flag, but adjust the implementation in
> the allocator (different from what you posted in previous version) so that it
> only looks at the flag after it tries to allocate from pcplist and finds out
> it's empty. So, no inventing of new page allocator entry points or checks such
> as the one you wrote above, but adding the new gfp flag in a way that it doesn't
> affect existing fast paths.

I like the idea of a new flag too :) After all telling the allocator more
about what your context can tolerate, via GFP flags, is not without
precedent.

thanks,

 - Joel