linux-kernel - Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number of CPUs

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Date:   Fri, 5 Apr 2019 13:49:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Alexey Dobriyan <adobriyan@...il.com>, mingo@...hat.com,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] sched/core: expand sched_getaffinity(2) to return number
 of CPUs

On Fri, Apr 05, 2019 at 01:08:58PM +0200, Florian Weimer wrote:
> * Peter Zijlstra:
> 
> > On Fri, Apr 05, 2019 at 12:16:39PM +0200, Florian Weimer wrote:
> >
> >> > True; but I suppose glibc already does lots of that anyway, right? It
> >> > does contain the right information.
> >> 
> >> If I recall correctly my last investigation,
> >> /sys/devices/system/cpu/possible does not reflect the size of the
> >> affinity mask, either.
> >
> > Strictly speaking correct; the bitmap can be longer than the highest
> > possible cpu number, however the remainder would be 0-padding and could
> > thus be stripped without issue.
> 
> Doesn't the kernel still enforce the larget bitmap in sched_getaffinity,
> even if the bits are always zero?

Oh crap, you're right. That's unfortunate I suppose.