Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

[Paul E. McKenney]

On Tue, Aug 11, 2020 at 04:44:21PM +0200, Thomas Gleixner wrote:
> Michal Hocko <mhocko@xxxxxxxx> writes:
> > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote:
> >> > On Sun 09-08-20 22:43:53, Uladzislau Rezki (Sony) wrote:
> >> > Is there any fundamental problem to make zone raw_spin_lock?
> >> > 
> >> Good point. Converting a regular spinlock to the raw_* variant can solve 
> >> an issue and to me it seems partly reasonable. Because there are other
> >> questions if we do it:
> >> 
> >> a) what to do with kswapd and "wake-up path" that uses sleepable lock:
> >>     wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait).
> >
> > If there is no RT friendly variant for waking up process from the atomic
> > context then we might need to special case this for the RT tree.
> 
> That's a solvable problem.
> 
> >> b) How RT people reacts on it? I guess they will no be happy.
> >
> > zone->lock should be held for a very limited amount of time.
> 
> Emphasis on should. free_pcppages_bulk() can hold it for quite some time
> when a large amount of pages are purged. We surely would have converted
> it to a raw lock long time ago otherwise.
> 
> For regular enterprise stuff a few hundred microseconds might qualify as
> a limited amount of time. For advanced RT applications that's way beyond
> tolerable..
> 
> >> As i described before, calling the __get_free_page(0) with 0 as argument
> >> will solve the (a). How correctly is it? From my point of view the logic
> >> that bypass the wakeup path should be explicitly defined.
> >
> > gfp_mask == 0 is GFP_NOWAIT (aka an atomic allocation request) which
> > doesn't wake up kswapd. So if the wakeup is a problem then this would be
> > a way to go.
> 
> The wakeup is the least of my worries.
> 
> > To me it would make more sense to support atomic allocations also for
> > the RT tree. Having both GFP_NOWAIT and GFP_ATOMIC which do not really
> > work for atomic context in RT sounds subtle and wrong.
> 
> Well, no. RT moves almost everything out of atomic context which means
> that GFP_ATOMIC is pretty meanlingless on a RT kernel. RT sacrifies
> performance for determinism. It's a known tradeoff.
> 
> Now RCU creates a new thing which enforces to make page allocation in
> atomic context possible on RT. What for?
> 
> What's the actual use case in truly atomic context for this new thing on
> an RT kernel?

It is not just RT kernels.  CONFIG_PROVE_RAW_LOCK_NESTING=y propagates
this constraint to all configurations, and a patch in your new favorite
subsystem really did trigger this lockdep check in a non-RT kernel.

> The actual RCU code disabling interrupts is an implementation detail
> which can easily be mitigated with a local lock.

In this case, we are in raw-spinlock context on entry to kfree_rcu().

							Thanx, Paul