[PATCH v2 1/1] cpumask: Don't waste memory for sysfs cpulist nodes

Andy Shevchenko
2022-09-22T19:49:52Z

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.

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