On Tue, Aug 04, 2020 at 06:12:03PM +0100, Matthew Wilcox wrote: > On Tue, Aug 04, 2020 at 07:02:14PM +0200, Vlastimil Babka wrote: > > > 2) There was a proposal from Matthew Wilcox: https://lkml.org/lkml/2020/7/31/1015 > > > > > > <snip> > > > On non-RT, we could make that lock a raw spinlock. On RT, we could > > > decline to take the lock. We'd need to abstract the spin_lock() away > > > behind zone_lock(zone), but that should be OK. > > > <snip> > > > > > > It would be great to use any existing flag, say GFP_NOWAIT. Suppose we > > > decline to take the lock across the page allocator for RT. But there is > > > at least one path that does it outside of the page allocator. GFP_NOWAIT > > > can wakeup the kswapd, whereas a "wake-up path" uses sleepable lock: > > > > > > wakeup_kswapd() -> wake_up_interruptible(&pgdat->kswapd_wait). > > > > > > Probably it can be fixed by the excluding of waking of the kswapd process > > > defining something like below: > > > > Is something missing here? > > > > > what is equal to zero and i am not sure if __get_free_page(0) handles > > > all that correctly, though it allocates and seems working on my test > > > machine! Please note it is related to "if we can reuse existing flags". > > > > > > In the meantime, please see below for a patch that adds a __GFP_FAST_TRY, > > > which can at least serve as a baseline against which other proposals can > > > be compared. The patch is based on the 5.8.0-rc3. > > > > > > Please RFC. > > > > At first glance __GFP_FAST_TRY (more descriptive name? __GFP_NO_LOCKS?) seems > > better than doing weird things with GFP_NOWAIT, but depends on the real benefits > > (hence my first questions). > > I think what Vlad is trying to say is that even GFP_NOWAIT will wake > kswapd, which involves taking a spinlock. If you specify 0 in your GFP > flags, then we won't wake kswapd. So a simple: > > #define GFP_NOLOCKS 0 > > should do the trick (modulo various casting, blah blah blah) > Yep, i meant that. Thank you Matthew! -- Vlad Rezki
