On Tue, Nov 07, 2023 at 03:08:02PM -0800, Ankur Arora wrote: > There are broadly three sets of uses of cond_resched(): > > 1. Calls to cond_resched() out of the goodness of our heart, > otherwise known as avoiding lockup splats. > > 2. Open coded variants of cond_resched_lock() which call > cond_resched(). > > 3. Retry or error handling loops, where cond_resched() is used as a > quick alternative to spinning in a tight-loop. > > When running under a full preemption model, the cond_resched() reduces > to a NOP (not even a barrier) so removing it obviously cannot matter. > > But considering only voluntary preemption models (for say code that > has been mostly tested under those), for set-1 and set-2 the > scheduler can now preempt kernel tasks running beyond their time > quanta anywhere they are preemptible() [1]. Which removes any need > for these explicitly placed scheduling points. > > The cond_resched() calls in set-3 are a little more difficult. > To start with, given it's NOP character under full preemption, it > never actually saved us from a tight loop. > With voluntary preemption, it's not a NOP, but it might as well be -- > for most workloads the scheduler does not have an interminable supply > of runnable tasks on the runqueue. > > So, cond_resched() is useful to not get softlockup splats, but not > terribly good for error handling. Ideally, these should be replaced > with some kind of timed or event wait. > For now we use cond_resched_stall(), which tries to schedule if > possible, and executes a cpu_relax() if not. > > All of these are from set-1 except for the retry loops in > task_function_call() or the mutex testing logic. > > Replace these with cond_resched_stall(). The others can be removed. > > [1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@xxxxxxxxxx/ > > Cc: Tejun Heo <tj@xxxxxxxxxx> > Cc: Zefan Li <lizefan.x@xxxxxxxxxxxxx> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > Cc: Peter Oberparleiter <oberpar@xxxxxxxxxxxxx> > Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx> > Cc: Will Deacon <will@xxxxxxxxxx> > Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx> > Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> > Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx> Sounds like the sort of test which should be put into linux-next to get test coverage right away. To see what really blows up. Luis
