On Thu 13-08-20 15:22:00, Thomas Gleixner wrote: > Uladzislau Rezki <urezki@xxxxxxxxx> writes: > > On Thu, Aug 13, 2020 at 09:50:27AM +0200, Michal Hocko wrote: > >> On Wed 12-08-20 02:13:25, Thomas Gleixner wrote: > >> [...] > >> > I can understand your rationale and what you are trying to solve. So, if > >> > we can actually have a distinct GFP variant: > >> > > >> > GFP_I_ABSOLUTELY_HAVE_TO_DO_THAT_AND_I_KNOW_IT_CAN_FAIL_EARLY > >> > >> Even if we cannot make the zone->lock raw I would prefer to not > >> introduce a new gfp flag. Well we can do an alias for easier grepping > >> #define GFP_RT_SAFE 0 > > Just using 0 is sneaky but yes, that's fine :) > > Bikeshedding: GFP_RT_NOWAIT or such might be more obvious. Sounds goood. > >> that would imply nowait semantic and would exclude waking up kswapd as > >> well. If we can make wake up safe under RT then the alias would reflect > >> that without any code changes. > > It basically requires to convert the wait queue to something else. Is > the waitqueue strict single waiter? I would have to double check. From what I remember only kswapd should ever sleep on it. > >> The second, and the more important part, would be to bail out anytime > >> the page allocator is to take a lock which is not allowed in the current > >> RT context. Something like > > >> + /* > >> + * Hard atomic contexts are not supported by the allocator for > >> + * anything but pcp requests > >> + */ > >> + if (!preemtable()) > > If you make that preemtible() it might even compile, but that still wont > work because if CONFIG_PREEMPT_COUNT=n then preemptible() is always > false. It would be nice to hide all that behind a helper and guarded by PREEMPT_RT. That would imply PREEMPT_COUNT automatically, right? > > So that should be: > > if (!preemptible() && gfp == GFP_RT_NOWAIT) > > which is limiting the damage to those callers which hand in > GFP_RT_NOWAIT. > > lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits > zone->lock in the wrong context. And we want to know about that so we > can look at the caller and figure out how to solve it. Yes, that would have to somehow need to annotate the zone_lock to be ok in those paths so that lockdep doesn't complain. -- Michal Hocko SUSE Labs
