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Date: Fri, 23 Sep 2022 08:23:09 -0700
From: Yury Norov <yury.norov@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
linux-kernel@...r.kernel.org,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Phil Auld <pauld@...hat.com>,
Petr Štetiar <ynezz@...e.cz>
Subject: Re: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist
nodes
On Fri, Sep 23, 2022 at 08:30:45AM +0200, Greg Kroah-Hartman wrote:
> On Thu, Sep 22, 2022 at 01:41:40PM -0700, Yury Norov wrote:
> > + Petr Štetiar <ynezz@...e.cz>,
> > + Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> >
> > On Thu, Sep 22, 2022 at 10:49:54PM +0300, Andy Shevchenko wrote:
> > > Currently the approximation is used which wastes the more memory
> > > the more CPUs are present on the system. Proposed change calculates
> > > the exact maximum needed in the worst case:
> > >
> > > NR_CPUS old new
> > > ------- --- ---
> > > 1 .. 1170 4096 4096
> > > 1171 .. 1860 4098 .. 6510 4096
> > > ... ... ...
> > > 2*4096 28672 19925
> > > 4*4096 57344 43597
> > > 8*4096 114688 92749
> > > 16*4096 229376 191053
> > > 32*4096 458752 403197
> > > 64*4096 917504 861949
> > > 128*4096 1835008 1779453
> > > 256*4096 3670016 3670016
> > >
> > > Under the hood the reccurent formula is being used:
> > > (5 - 0) * 2 +
> > > (50 - 5) * 3 +
> > > (500 - 50) * 4 +
> > > (5000 - 500) * 5 +
> > > ...
> > > (X[i] - X[i-1]) * i
> > >
> > > which allows to count the exact maximum length in the worst case,
> > > i.e. when each second CPU is being listed. For backward compatibility
> > > for more than 1170 and less than 1861 CPUs the page size is preserved.
> > >
> > > For less than 1171 and more than 1 million CPUs the old is being used.
> >
> > 1861
> >
> > >
> > > Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> > > ---
> > > v2: described better the advantage for 1171..1860 CPUs cases
> > > include/linux/cpumask.h | 48 +++++++++++++++++++++++++++++++++++++++++
> > > 1 file changed, 48 insertions(+)
> > >
> > > diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> > > index 1b442fb2001f..12cf0905ca74 100644
> > > --- a/include/linux/cpumask.h
> > > +++ b/include/linux/cpumask.h
> > > @@ -1122,6 +1122,21 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
> > > *
> > > * for cpumap NR_CPUS * 9/32 - 1 should be an exact length.
> > > *
> > > + * for cpulist the reccurent formula is being used:
> > > + * (5 - 0) * 2 +
> > > + * (50 - 5) * 3 +
> > > + * (500 - 50) * 4 +
> > > + * (5000 - 500) * 5 +
> > > + * ...
> > > + * (X[i] - X[i-1]) * i
> > > + *
> > > + * which allows to count the exact maximum length in the worst case,
> > > + * i.e. when each second CPU is being listed. For backward compatibility
> > > + * for more than 1170 and less than 1861 CPUs the page size is preserved.
> > > + *
> > > + * For less than 1171 and more than 1 million CPUs the old is being used
> > > + * as described below:
> > > + *
> > > * For cpulist 7 is (ceil(log10(NR_CPUS)) + 1) allowing for NR_CPUS to be up
> > > * to 2 orders of magnitude larger than 8192. And then we divide by 2 to
> > > * cover a worst-case of every other cpu being on one of two nodes for a
> > > @@ -1132,6 +1147,39 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
> > > */
> > > #define CPUMAP_FILE_MAX_BYTES (((NR_CPUS * 9)/32 > PAGE_SIZE) \
> > > ? (NR_CPUS * 9)/32 - 1 : PAGE_SIZE)
> > > +
> > > +#define __CPULIST_FOR_10(x) (((x + 1) / 2 - 0) * 2)
> > > +#define __CPULIST_FOR_100(x) (((x + 1) / 2 - 5) * 3)
> > > +#define __CPULIST_FOR_1000(x) (((x + 1) / 2 - 50) * 4)
> > > +#define __CPULIST_FOR_10000(x) (((x + 1) / 2 - 500) * 5)
> > > +#define __CPULIST_FOR_100000(x) (((x + 1) / 2 - 5000) * 6)
> > > +#define __CPULIST_FOR_1000000(x) (((x + 1) / 2 - 50000) * 7)
> >
> > The defs below will be nicer if you make it like this:
> >
> > #define __CPULIST_FOR_10(x) (((x + 1) / 2 - 0) * 2)
> > #define __CPULIST_FOR_100(x) __CPULIST_FOR_10(10) + (((x + 1) / 2 - 5) * 3)
> > #define __CPULIST_FOR_1000(x) __CPULIST_FOR_100(100) + (((x + 1) / 2 - 50) * 4)
> > ...
> >
> >
> >
> > > +#if NR_CPUS < 1861
> > > +#define CPULIST_FILE_MAX_BYTES PAGE_SIZE
> >
> > The comment says:
> > for more than 1170 and less than 1861 CPUs the page size is preserved.
> >
> > Which doesn't look correct. Looks like it should be:
> > for less than 1861 CPUs the page size is preserved.
> >
> > Or I miss something?
> >
> > > +#elif NR_CPUS < 10000
> > > +#define CPULIST_FILE_MAX_BYTES \
> > > + (__CPULIST_FOR_10(10) + \
> > > + __CPULIST_FOR_100(100) + \
> > > + __CPULIST_FOR_1000(1000) + \
> > > + __CPULIST_FOR_10000(NR_CPUS))
> > > +#elif NR_CPUS < 100000
> > > +#define CPULIST_FILE_MAX_BYTES \
> > > + (__CPULIST_FOR_10(10) + \
> > > + __CPULIST_FOR_100(100) + \
> > > + __CPULIST_FOR_1000(1000) + \
> > > + __CPULIST_FOR_10000(10000) + \
> > > + __CPULIST_FOR_100000(NR_CPUS))
> > > +#elif NR_CPUS < 1000000
> > > +#define CPULIST_FILE_MAX_BYTES \
> > > + (__CPULIST_FOR_10(10) + \
> > > + __CPULIST_FOR_100(100) + \
> > > + __CPULIST_FOR_1000(1000) + \
> > > + __CPULIST_FOR_10000(10000) + \
> > > + __CPULIST_FOR_100000(100000) + \
> > > + __CPULIST_FOR_1000000(NR_CPUS))
> > > +#else
> > > #define CPULIST_FILE_MAX_BYTES (((NR_CPUS * 7)/2 > PAGE_SIZE) ? (NR_CPUS * 7)/2 : PAGE_SIZE)
> > > +#endif
> > >
> > > #endif /* __LINUX_CPUMASK_H */
> > > --
> > > 2.35.1
> >
> > I'm OK to take this in replace for Phil's version, but the commit that
> > introduces CPULIST_FILE_MAX_BYTES is already in mainline: 7ee951acd31a8
> > ("drivers/base: fix userspace break from using bin_attributes for cpumap
> > and cpulist"). Can you rebase it on top of v6.0-rc6?
> >
> > Greg, since Andy's version is more precise, I'd like to send a pull
> > request with it in -rc7. Can you drop Phil's patch so I'll go with
> > this one?
>
> Let me get this fix to Linus now for 6.0-final as it is hitting people
> right now. Making it "cleaner" after that is fine to go through your
> tree as that's less of an issue, right?
OK
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