Messages in this thread | | | Date | Thu, 22 Sep 2022 20:38:06 -0400 | From | Phil Auld <> | Subject | Re: [PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist nodes |
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Hi Andy,
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.
The memory is not really wasted since it's probably temporary in userspace and in the kernel it _is_ temporary and is only the length of the kasprintf string, which is most of the time much less.
But that said, it is more accurate than the previous estimate.
I was wondering if you were going to try to come up with a suitable compile time macro :)
I tested 2, 8192 and 16k since the kernel does not want to build for other reasons with NR_CPUS at 32k.
Reviewed-by: Phil Auld <pauld@redhat.com> Tested-by: Phil Auld <pauld@redhat.com>
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.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) > + > +#if NR_CPUS < 1861 > +#define CPULIST_FILE_MAX_BYTES PAGE_SIZE > +#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 >
Cheers, Phil
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