On Tue 11-08-20 11:37:13, Uladzislau Rezki wrote: > On Tue, Aug 11, 2020 at 10:19:17AM +0200, Michal Hocko wrote: > > On Mon 10-08-20 21:25:26, Michal Hocko wrote: > > > On Mon 10-08-20 18:07:39, Uladzislau Rezki wrote: > > [...] > > > > The problem that i see is we can not use the page allocator from atomic > > > > contexts, what is our case: > > > > > > > > <snip> > > > > local_irq_save(flags) or preempt_disable() or raw_spinlock(); > > > > __get_free_page(GFP_ATOMIC); > > > > <snip> > > > > > > > > So if we can convert the page allocator to raw_* lock it will be appreciated, > > > > at least from our side, IMHO, not from RT one. But as i stated above we need > > > > to sort raised questions out if converting is done. > > > > > > > > What is your view? > > > > > > 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. > > > > I was thinking about this some more. I still think the above would be a > > reasonable goal we should try to achieve. If for not other then for > > future maintainability (especially after the RT patchset is merged). > > I have tried to search for any known problems/attempts to make > > zone->lock raw but couldn't find anything. Maybe somebody more involved > > in RT world have something to say about that. > > > I tried yesterday to convert zone->lock. See below files i had to modify: > <snip> > modified: include/linux/mmzone.h > modified: mm/compaction.c > modified: mm/memory_hotplug.c > modified: mm/page_alloc.c > modified: mm/page_isolation.c > modified: mm/page_reporting.c > modified: mm/shuffle.c > modified: mm/vmscan.c > modified: mm/vmstat.c > <snip> > > There is one more lock, that is zone->lru_lock one. Both zone->lock and this > one intersect between each other. If the lru_lock can be nested under zone->lock > it should be converted as well. But i need to analyze it farther. There are > two wrapper functions which are used as common interface to lock/unlock both > locks. See compact_lock_irqsave()/compact_unlock_should_abort_lru() in the > mm/compaction.c. > > Any thoughts here? I am not an expert on compaction. Vlastimil would know better. My thinking was that zone->lock is a tail lock but compaction/page isolation might be doing something I am not aware of right now. > Anyway i tried to convert only zone->lock and use page allocator passing there > gfp_mask=0 as argument. So it works. CONFIG_PROVE_RAW_LOCK_NESTING does not > complain about any "bad" lock nesting. > > > Anyway, if the zone->lock is not a good fit for raw_spin_lock then the > > only way I can see forward is to detect real (RT) atomic contexts and > > bail out early before taking the lock in the allocator for NOWAIT/ATOMIC > > requests. > > > For RT kernel we can detect it for sure. preemtable() works just fine there, > i.e. we can identify the context we are currently in. In previous email I didn't mention why I prefer full NOWAIT semantic over rt specific bailouts. There are users making NOWAIT allocation attempts as an opportunistic allocation request which is OK to fail as they have a fallback to go through. This would imply they would prefer to know this ASAP rather then get blocked and sleep. A lack of reports for PREEMPT_RT would suggest that nobody has noticed as this though. -- Michal Hocko SUSE Labs
