On 03.09.24 14:21, Anders Roxell wrote:
Hi,
I've noticed that the futex01-thread-* tests in will-it-scale-sys-threads
are running about 2% slower on v6.10-rc1 compared to v6.9, and this
slowdown continues with v6.11-rc4. I am focused on identifying any
performance regressions greater than 2% that occur in automated
testing on arm64 HW.
Using git bisect, I traced the issue to commit
f002882ca369 ("mm: merge folio_is_secretmem() and
folio_fast_pin_allowed() into gup_fast_folio_allowed()").
Thanks for analyzing the (slight) regression!
My tests were performed on m7g.large and m7g.metal instances:
* The slowdown is consistent regardless of the number of threads;
futex1-threads-128 performs similarly to futex1-threads-2, indicating
there is no scalability issue, just a minor performance overhead.
* The test doesn’t involve actual futex operations, just dummy wake/wait
on a variable that isn’t accessed by other threads, so the results might
not be very significant.
Given that this seems to be a minor increase in code path length rather
than a scalability issue, would this be considered a genuine regression?
Likely not, I've seen these kinds of regressions (for example in my fork
micro-benchmarks) simply because the compiler slightly changes the code
layout, or suddenly decides to not inline a functions.
Still it is rather unexpected, so let's find out what's happening.
My first intuition would have been that the compiler now decides to not
inline gup_fast_folio_allowed() anymore, adding a function call.
LLVM seems to inline it for me. GCC not.
Would this return the original behavior for you?
diff --git a/mm/gup.c b/mm/gup.c
index 69c483e2cc32d..6642f09c95881 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2726,7 +2726,8 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
* in the fast path, so instead we whitelist known good cases and if in doubt,
* fall back to the slow path.
*/
-static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
+static __always_inline bool gup_fast_folio_allowed(struct folio *folio,
+ unsigned int flags)
{
bool reject_file_backed = false;
struct address_space *mapping;
--
Cheers,
David / dhildenb
