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[94.245.46.49]) by smtp.gmail.com with ESMTPSA id n3sm5865720ljj.39.2020.08.04.12.47.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 04 Aug 2020 12:47:40 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 4 Aug 2020 21:47:38 +0200 To: Matthew Wilcox Cc: Vlastimil Babka , "Uladzislau Rezki (Sony)" , LKML , RCU , linux-mm@kvack.org, Andrew Morton , "Paul E . McKenney" , "Theodore Y . Ts'o" , Joel Fernandes , Sebastian Andrzej Siewior , Oleksiy Avramchenko Subject: Re: [RFC-PROTOTYPE 1/1] mm: Add __GFP_FAST_TRY flag Message-ID: <20200804194738.GB29837@pc636> References: <20200803163029.1997-1-urezki@gmail.com> <1d50a46a-b97f-96b2-8a5c-21075f022f01@suse.cz> <20200804171203.GH23808@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200804171203.GH23808@casper.infradead.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Rspamd-Queue-Id: 1695618140B70 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam01 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 > > > > > > > > > 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. > > > > > > > > > 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