Re: [RFC PATCH 57/86] coccinelle: script to remove cond_resched()

[Paul E. McKenney]

On Tue, Nov 07, 2023 at 03:07:53PM -0800, Ankur Arora wrote:
> Rudimentary script to remove the straight-forward subset of
> cond_resched() and allies:
> 
> 1)  if (need_resched())
> 	  cond_resched()
> 
> 2)  expression*;
>     cond_resched();  /* or in the reverse order */
> 
> 3)  if (expression)
> 	statement
>     cond_resched();  /* or in the reverse order */
> 
> The last two patterns depend on the control flow level to ensure
> that the complex cond_resched() patterns (ex. conditioned ones)
> are left alone and we only pick up ones which are only minimally
> related the neighbouring code.

This series looks to get rid of stall warnings for long in-kernel
preempt-enabled code paths, which is of course a very good thing.
But removing all of the cond_resched() calls can actually increase
scheduling latency compared to the current CONFIG_PREEMPT_NONE=y state,
correct?

If so, it would be good to take a measured approach.  For example, it
is clear that a loop that does a cond_resched() every (say) ten jiffies
can remove that cond_resched() without penalty, at least in kernels built
with either CONFIG_NO_HZ_FULL=n or CONFIG_PREEMPT=y.  But this is not so
clear for a loop that does a cond_resched() every (say) ten microseconds.

Or am I missing something here?

							Thanx, Paul

> Cc: Julia Lawall <Julia.Lawall@xxxxxxxx>
> Cc: Nicolas Palix <nicolas.palix@xxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
>  scripts/coccinelle/api/cond_resched.cocci | 53 +++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
>  create mode 100644 scripts/coccinelle/api/cond_resched.cocci
> 
> diff --git a/scripts/coccinelle/api/cond_resched.cocci b/scripts/coccinelle/api/cond_resched.cocci
> new file mode 100644
> index 000000000000..bf43768a8f8c
> --- /dev/null
> +++ b/scripts/coccinelle/api/cond_resched.cocci
> @@ -0,0 +1,53 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/// Remove naked cond_resched() statements
> +///
> +//# Remove cond_resched() statements when:
> +//#   - executing at the same control flow level as the previous or the
> +//#     next statement (this lets us avoid complicated conditionals in
> +//#     the neighbourhood.)
> +//#   - they are of the form "if (need_resched()) cond_resched()" which
> +//#     is always safe.
> +//#
> +//# Coccinelle generally takes care of comments in the immediate neighbourhood
> +//# but might need to handle other comments alluding to rescheduling.
> +//#
> +virtual patch
> +virtual context
> +
> +@ r1 @
> +identifier r;
> +@@
> +
> +(
> + r = cond_resched();
> +|
> +-if (need_resched())
> +-	cond_resched();
> +)
> +
> +@ r2 @
> +expression E;
> +statement S,T;
> +@@
> +(
> + E;
> +|
> + if (E) S
> +|
> + if (E) S else T
> +|
> +)
> +-cond_resched();
> +
> +@ r3 @
> +expression E;
> +statement S,T;
> +@@
> +-cond_resched();
> +(
> + E;
> +|
> + if (E) S
> +|
> + if (E) S else T
> +)
> -- 
> 2.31.1
>