Re: [PATCH v2] sched/psi: use __ffs() to walk task-count bitmasks in psi_group_change()

[Usama Arif <usama.arif@linux.dev>]

On Fri, 17 Jul 2026 03:59:39 -0700 Usama Arif <usama.arif@linux.dev> wrote:

> psi_group_change() walks the @clear and @set bitmasks to
> decrement/increment groupc->tasks[t]. Both masks are at most
> NR_PSI_TASK_COUNTS (=4) wide and typically have one or two bits
> set. Today's form visits every position up to the highest set bit:
> 
> 	for (t = 0, m = clear; m; m &= ~(1 << t), t++) {
> 		if (!(m & (1 << t)))
> 			continue;
> 		...
> 	}
> 
> so a mask with only bit 3 set still spins four times through the
> skip path. Switch both walks to __ffs() + m &= m-1 form:
> 
> 	while (clear) {
> 		t = __ffs(clear);
> 		clear &= clear - 1;
> 		...
> 	}
> 
> which iterates only over the set bits and terminates naturally on
> m == 0. m & (m - 1) clears the lowest set bit. This code is easier
> to read as well.
> 
> An in-kernel microbench (noinline, same body, IRQs off, pinned CPU
> on Zen4c, min-of-10 cyc/call) over mask distributions produced by
> common scheduler PSI paths:
> 
>   mask pattern                        old    new    delta
>   empty          (clear=0x0, set=0x0)  3.68   3.68    +0%
>   sleep          (clear=0x4, set=0x0)  9.60   3.74   -61%
>   iowait-sleep   (clear=0x4, set=0x1) 12.32   4.45   -63%
>   memstall-sleep (clear=0xc, set=0x0) 11.87   5.54   -53%
>   wake           (clear=0x0, set=0x4)  7.10   3.70   -47%
>   iowait-wake    (clear=0x1, set=0x4) 10.83   4.48   -58%
> 
> Every non-empty case wins 47-63%: old cost tracks the highest set bit
> (linear walk), new cost tracks the count of set bits (skip zeros via
> TZCNT). Single-bit patterns run at the empty-case floor.
> 
> The generated psi_group_change() text also shrinks by 67 bytes under
> -O2 -march=x86-64 (756 -> 689): no scratch register for a "constant 1"
> (only __ffs's operand is needed), simpler bit-clear (LEA+AND vs
> SHL+NOT+AND after the test), and no skip-if-unset check per position.
> 
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>


Hello Peter!

Just wanted to check if there were any comments or feedback on this patch?

I checked sched/core branch and didn't see the patch there.

Thanks!
Usama