On Tue, Nov 07, 2023 at 03:08:07PM -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. > > Almost all the cond_resched() calls are from set-1. Remove them. > > [1] https://lore.kernel.org/lkml/20231107215742.363031-1-ankur.a.arora@xxxxxxxxxx/ > > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> > Cc: Kees Cook <keescook@xxxxxxxxxxxx> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx> > Cc: Thomas Graf <tgraf@xxxxxxx> > Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx> > --- > lib/crc32test.c | 2 -- > lib/crypto/mpi/mpi-pow.c | 1 - > lib/memcpy_kunit.c | 5 ----- > lib/random32.c | 1 - > lib/rhashtable.c | 2 -- > lib/test_bpf.c | 3 --- > lib/test_lockup.c | 2 +- > lib/test_maple_tree.c | 8 -------- > lib/test_rhashtable.c | 10 ---------- > 9 files changed, 1 insertion(+), 33 deletions(-) Nack. -- Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
