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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Thomas Gleixner writes: > Paul! > > On Tue, Nov 21 2023 at 07:19, Paul E. McKenney wrote: >> On Tue, Nov 21, 2023 at 10:00:59AM -0500, Steven Rostedt wrote: >>> Right now, the use of cond_resched() is basically a whack-a-mole game where >>> we need to whack all the mole loops with the cond_resched() hammer. As >>> Thomas said, this is backwards. It makes more sense to just not preempt in >>> areas that can cause pain (like holding a mutex or in an RCU critical >>> section), but still have the general kernel be fully preemptable. >> >> Which is quite true, but that whack-a-mole game can be ended without >> getting rid of build-time selection of the preemption model. Also, >> that whack-a-mole game can be ended without eliminating all calls to >> cond_resched(). > > Which calls to cond_resched() should not be eliminated? > > They all suck and keeping some of them is just counterproductive as > again people will sprinkle them all over the place for the very wrong > reasons. And, as Thomas alludes to here, cond_resched() is not always cost free. Needing to call cond_resched() forces us to restructure hot paths in ways that results in worse performance/complex code. One example is clear_huge_page(), where removing the need to call cond_resched() every once in a while allows the processor to optimize differently. *Milan* mm/clear_huge_page x86/clear_huge_page change (GB/s) (GB/s) pg-sz=2MB 14.55 19.29 +32.5% pg-sz=1GB 19.34 49.60 +156.4% (See https://lore.kernel.org/all/20230830184958.2333078-1-ankur.a.arora@oracle.com/) And, that's one of the simpler examples from mm. We do this kind of arbitrary batching all over the place. Or see the filemap_read() example that Linus gives here: https://lore.kernel.org/lkml/CAHk-=whpYjm_AizQij6XEfTd7xvGjrVCx5gzHcHm=2Xijt+Kyg@mail.gmail.com/#t Thanks -- ankur