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
