linux-kernel - Re: [PATCH v2] clocksource: Replace loop within clocks_calc_mult_shift() with clz-based calculation for sftacc

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Message-ID: <aEkYTq5JdwGwWcCF@vaxr-BM6660-BM6360>
Date: Wed, 11 Jun 2025 13:46:54 +0800
From: I Hsin Cheng <richard120310@...il.com>
To: John Stultz <jstultz@...gle.com>
Cc: tglx@...utronix.de, sboyd@...nel.org, linux-kernel@...r.kernel.org,
	skhan@...uxfoundation.org, linux-kernel-mentees@...ts.linux.dev,
	jserv@...s.ncku.edu.tw
Subject: Re: [PATCH v2] clocksource: Replace loop within
 clocks_calc_mult_shift() with clz-based calculation for sftacc

On Tue, Jun 10, 2025 at 09:31:49PM -0700, John Stultz wrote:
> On Tue, Jun 10, 2025 at 8:30 PM I Hsin Cheng <richard120310@...il.com> wrote:
> >
> > v1 -> v2:
> >         - Refine commit message to explain more about "why"
> >         - Check the frequency of "clocks_calc_mult_shift()" get called,
> >           it's not in hotpath on my machine, refine the commit message
> >       to avoid overselling it
> >         - Add comments for the code to explain the implementation in
> >           more detail
> >         - Handle case for "tmp == 0" to avoid undefined behavior
> >     - Experiment using ffs() but it's used for finding the LSB of
> >       a number, which isn't MSB as we want. It would still need
> >       looping when intended to use ffs() in this scenario
> 
> Oh, apologies for mixing that up and leading you astray!
> 
> 
> > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
> > index 2a7802ec480c..59145d762f1e 100644
> > --- a/kernel/time/clocksource.c
> > +++ b/kernel/time/clocksource.c
> > @@ -66,10 +66,20 @@ clocks_calc_mult_shift(u32 *mult, u32 *shift, u32 from, u32 to, u32 maxsec)
> >          * range:
> >          */
> >         tmp = ((u64)maxsec * from) >> 32;
> > -       while (tmp) {
> > -               tmp >>=1;
> > -               sftacc--;
> > -       }
> > +
> > +       /*
> > +        * Decrement "sftacc" by the number of bits needed to represent "tmp".
> > +        * Using (32 - __builtin_clz(tmp)) to ge the bit width:
> > +        * - __builtin_clz(tmp) returns the count of leading zero bits
> > +        * - (32 - __builtin_clz(tmp)) gives us the number of significant bits
> > +        *
> > +        * Note: It use 32 instead of 31 because we want bit width (0-index MSB
> > +        * position + 1), not just the MSB position itself.
> > +        *
> > +        * Handle tmp == 0 separately since __builtin_clz(0) has undefined behavior.
> > +        */
> > +       if (tmp)
> > +               sftacc -= (32 - __builtin_clz(tmp));
> 
> Still think the detail that __builtin_clz only works on the bottom
> 32bits is good to highlight in the comment.
> Though maybe, would explicitly casting tmp to a u32 help it be more clear here?
> 
> thanks
> -john
>

Hi John,

Sure thing, sorry I missed the part to mention it onlys works on the
lower 32 bits of "tmp". I'll amend that ASAP.

Btw I just realized it's more appropriate to use "find_last_bit()"
instead of __builtin_clz(). Most of the in-tree use cases utilize
find_last_bit() as it support more optimization. I'll cc to bitmap's
maintainer in the next version and see what suggestion he gives so we
can make sure we consider everything as much as possible.

Thanks !

Best regards,
I Hsin Cheng