Re: [PATCH 13/16] perf: Use sysfs_emit() for cpumask show callbacks

[Yury Norov <yury.norov@gmail.com>]

On Fri, May 29, 2026 at 01:06:19PM +0100, David Laight wrote:
> On Fri, 29 May 2026 12:05:08 +0100
> Robin Murphy <robin.murphy@arm.com> wrote:
> 
> > On 2026-05-28 7:36 pm, Yury Norov wrote:
> > > These callbacks are sysfs show paths.
> > > 
> > > Use sysfs_emit() and cpumask_pr_args() to emit the masks.
> > > 
> > > This prepares for removing cpumap_print_to_pagebuf().  
> > 
> > TBH, looking at this diff I think it only shows the value of having a 
> > helper to abstract the boilerplate...
> > 
> > I'm not sure I agree with the argument of removing something entirely 
> > just because it may occasionally be misused, but could we at least have 
> > something like:
> > 
> > #define sysfs_emit_cpumask(buf, mask) \
> > 	sysfs_emit((buf), "%*pbl\n", cpumask_pr_args(mask))
> >
> > to save the mess in all the many places where the current 
> > cpumap_print_to_pagebuf() usage _is_ entirely appropriate?

This way you have to add 2 wrappers:

 #define sysfs_emit_cpulist(buf, mask) \
 	sysfs_emit((buf), "%*pbl\n", cpumask_pr_args(mask))

 and

 #define sysfs_emit_cpumask(buf, mask) \
 	sysfs_emit((buf), "%*pb\n", cpumask_pr_args(mask))

There are people who complain even about DIV_ROUND_UP(), how hard it is
to keep all that helpers in memory, and all that things.

https://lore.kernel.org/all/20260304124805.GB2277644@noisy.programming.kicks-ass.net/

Disagree about DIV_ROUND_UP() (because yeah, I'm bad in math), but
this sysfs_emit_cpumask() is a complete syntax redundancy.

Once we have it, people will do this type of things:

        tmp = kmalloc(PAGE_SIZE);
        sysfs_emit_cpumask(tmp, mask);
        sysfs_emit(buf, "my prefix: %s\n", tmp);
        kfree(tmp);

Patch #1 in this series is one example. My series that removes
bitmap_print_to_pagebuf() will give you more:

https://lore.kernel.org/all/20260303200842.124996-2-ynorov@nvidia.com/

It doesn't mean that *you* will misuse the API. It means that *I* will
have to inspect the codebase for that type of bugs periodically.

So, the overall state is simple: we've got well-established
printf()-like functions that people know and understand, and we also
have exotic APIs here and there with a non-standard interface and a
clear potential to misuse. In this case, they have historical roots,
but now we don't need them.

> That has the advantage of letting you change how it is done (again)
> without having to find all the callers.

You mean things like silencing the prints or adding a prefix?

If you believe that perf subsystem would benefit from it - that's
OK. Just please keep it local. The kernel globally doesn't need to
'change how it is done' beyond the lib/vsprintf.  The kernel really
needs people to use something that the other people are familiar with.

Thanks,
Yury