Re: [RFC PATCH 71/86] treewide: lib: remove cond_resched()

[Herbert Xu]

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