Re: [GIT PULL] Introduce try_alloc_pages for 6.15

[Johannes Weiner <hannes@cmpxchg.org>]

On Sun, Mar 30, 2025 at 02:30:15PM -0700, Alexei Starovoitov wrote:
> On Sun, Mar 30, 2025 at 1:42 PM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> > On Thu, 27 Mar 2025 at 07:52, Alexei Starovoitov
> > <alexei.starovoitov@gmail.com> wrote:
> > >
> > > The pull includes work from Sebastian, Vlastimil and myself
> > > with a lot of help from Michal and Shakeel.
> > > This is a first step towards making kmalloc reentrant to get rid
> > > of slab wrappers: bpf_mem_alloc, kretprobe's objpool, etc.
> > > These patches make page allocator safe from any context.
> >
> > So I've pulled this too, since it looked generally fine.
> 
> Thanks!
> 
> > The one reaction I had is that when you basically change
> >
> >         spin_lock_irqsave(&zone->lock, flags);
> >
> > into
> >
> >         if (!spin_trylock_irqsave(&zone->lock, flags)) {
> >                 if (unlikely(alloc_flags & ALLOC_TRYLOCK))
> >                         return NULL;
> >                 spin_lock_irqsave(&zone->lock, flags);
> >         }
> >
> > we've seen bad cache behavior for this kind of pattern in other
> > situations: if the "try" fails, the subsequent "do the lock for real"
> > case now does the wrong thing, in that it will immediately try again
> > even if it's almost certainly just going to fail - causing extra write
> > cache accesses.
> >
> > So typically, in places that can see contention, it's better to either do
> >
> >  (a) trylock followed by a slowpath that takes the fact that it was
> > locked into account and does a read-only loop until it sees otherwise
> >
> >      This is, for example, what the mutex code does with that
> > __mutex_trylock() -> mutex_optimistic_spin() pattern, but our
> > spinlocks end up doing similar things (ie "trylock" followed by
> > "release irq and do the 'relax loop' thing).
> 
> Right,
> __mutex_trylock(lock) -> mutex_optimistic_spin() pattern is
> equivalent to 'pending' bit spinning in qspinlock.
> 
> > or
> >
> >  (b) do the trylock and lock separately, ie
> >
> >         if (unlikely(alloc_flags & ALLOC_TRYLOCK)) {
> >                 if (!spin_trylock_irqsave(&zone->lock, flags))
> >                         return NULL;
> >         } else
> >                 spin_lock_irqsave(&zone->lock, flags);
> >
> > so that you don't end up doing two cache accesses for ownership that
> > can cause extra bouncing.
> 
> Ok, I will switch to above.
> 
> > I'm not sure this matters at all in the allocation path - contention
> > may simply not be enough of an issue, and the trylock is purely about
> > "unlikely NMI worries", but I do worry that you might have made the
> > normal case slower.
> 
> We actually did see zone->lock being contended in production.
> Last time the culprit was an inadequate per-cpu caching and
> these series in 6.11 fixed it:
> https://lwn.net/Articles/947900/
> I don't think we've seen it contended in the newer kernels.
>
> Johannes, pls correct me if I'm wrong.

Contention should indeed be rare in practice. This has become a very
coarse lock, with nowadays hundreds of HW threads hitting still only
one or two zones. A lot rides on the fastpath per-cpu caches, and it
becomes noticable very quickly if those are sized inappropriately.

> But to avoid being finger pointed, I'll switch to checking alloc_flags
> first. It does seem a better trade off to avoid cache bouncing because
> of 2nd cmpxchg. Though when I wrote it this way I convinced myself and
> others that it's faster to do trylock first to avoid branch misprediction.

If you haven't yet, it could be interesting to check if/where branches
are generated at all, given the proximity and the heavy inlining
between where you pass the flag and where it's tested.