On Tue, Nov 28, 2023 at 10:30:53AM -0800, Ankur Arora wrote: > > Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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@xxxxxxxxxx/) > > 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@xxxxxxxxxxxxxx/#t I already agree that some cond_resched() calls can cause difficulties. But that is not the same as proving that they *all* should be removed. Thanx, Paul
