| Message ID | 20231010145549.1244748-1-willy@infradead.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | bounds: Support non-power-of-two CONFIG_NR_CPUS | expand |
On Tue, 2023-10-10 at 15:55 +0100, Matthew Wilcox (Oracle) wrote: > ilog2() rounds down, so for example when PowerPC 85xx sets > CONFIG_NR_CPUS > to 24, we will only allocate 4 bits to store the number of CPUs > instead > of 5. Use bits_per() instead, which rounds up. Found by code > inspection. > The effect of this would probably be a misaccounting when doing NUMA > balancing, so to a user, it would only be a performance penalty. > The effects may be more wide-spread; it's hard to tell. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > Fixes: 90572890d202 ("mm: numa: Change page last {nid,pid} into > {cpu,pid}") > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Mel Gorman <mgorman@techsingularity.net> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Rik van Riel <riel@surriel.com> Reviewed-by: Rik van Riel <riel@surriel.com>
On Tue, Oct 10, 2023 at 03:55:49PM +0100, Matthew Wilcox (Oracle) wrote: > ilog2() rounds down, so for example when PowerPC 85xx sets CONFIG_NR_CPUS > to 24, we will only allocate 4 bits to store the number of CPUs instead > of 5. Use bits_per() instead, which rounds up. Found by code inspection. > The effect of this would probably be a misaccounting when doing NUMA > balancing, so to a user, it would only be a performance penalty. > The effects may be more wide-spread; it's hard to tell. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > Fixes: 90572890d202 ("mm: numa: Change page last {nid,pid} into {cpu,pid}") > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Mel Gorman <mgorman@techsingularity.net> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Rik van Riel <riel@surriel.com> Acked-by: Mel Gorman <mgorman@techsingularity.net>
diff --git a/kernel/bounds.c b/kernel/bounds.c index b529182e8b04..c5a9fcd2d622 100644 --- a/kernel/bounds.c +++ b/kernel/bounds.c @@ -19,7 +19,7 @@ int main(void) DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS); DEFINE(MAX_NR_ZONES, __MAX_NR_ZONES); #ifdef CONFIG_SMP - DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS)); + DEFINE(NR_CPUS_BITS, bits_per(CONFIG_NR_CPUS)); #endif DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t)); #ifdef CONFIG_LRU_GEN
ilog2() rounds down, so for example when PowerPC 85xx sets CONFIG_NR_CPUS to 24, we will only allocate 4 bits to store the number of CPUs instead of 5. Use bits_per() instead, which rounds up. Found by code inspection. The effect of this would probably be a misaccounting when doing NUMA balancing, so to a user, it would only be a performance penalty. The effects may be more wide-spread; it's hard to tell. Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Fixes: 90572890d202 ("mm: numa: Change page last {nid,pid} into {cpu,pid}") Cc: Peter Zijlstra <peterz@infradead.org> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Ingo Molnar <mingo@kernel.org> Cc: Rik van Riel <riel@surriel.com> --- kernel/bounds.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)