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 >
