On 12.02.24 11:56, David Hildenbrand wrote:
On 12.02.24 11:32, Ryan Roberts wrote:
On 12/02/2024 10:11, David Hildenbrand wrote:
Hi Ryan,
-static void tlb_batch_pages_flush(struct mmu_gather *tlb)
+static void __tlb_batch_free_encoded_pages(struct mmu_gather_batch *batch)
{
- struct mmu_gather_batch *batch;
-
- for (batch = &tlb->local; batch && batch->nr; batch = batch->next) {
- struct encoded_page **pages = batch->encoded_pages;
+ struct encoded_page **pages = batch->encoded_pages;
+ unsigned int nr, nr_pages;
+ /*
+ * We might end up freeing a lot of pages. Reschedule on a regular
+ * basis to avoid soft lockups in configurations without full
+ * preemption enabled. The magic number of 512 folios seems to work.
+ */
+ if (!page_poisoning_enabled_static() && !want_init_on_free()) {
Is the performance win really worth 2 separate implementations keyed off this?
It seems a bit fragile, in case any other operations get added to free which are
proportional to size in future. Why not just always do the conservative version?
I really don't want to iterate over all entries on the "sane" common case. We
already do that two times:
a) free_pages_and_swap_cache()
b) release_pages()
Only the latter really is required, and I'm planning on removing the one in (a)
to move it into (b) as well.
So I keep it separate to keep any unnecessary overhead to the setups that are
already terribly slow.
No need to iterate a page full of entries if it can be easily avoided.
Especially, no need to degrade the common order-0 case.
Yeah, I understand all that. But given this is all coming from an array, (so
easy to prefetch?) and will presumably all fit in the cache for the common case,
at least, so its hot for (a) and (b), does separating this out really make a
measurable performance difference? If yes then absolutely this optimizaiton
makes sense. But if not, I think its a bit questionable.
I primarily added it because
(a) we learned that each cycle counts during mmap() just like it does
during fork().
(b) Linus was similarly concerned about optimizing out another batching
walk in c47454823bd4 ("mm: mmu_gather: allow more than one batch of
delayed rmaps"):
"it needs to walk that array of pages while still holding the page table
lock, and our mmu_gather infrastructure allows for batching quite a lot
of pages. We may have thousands on pages queued up for freeing, and we
wanted to walk only the last batch if we then added a dirty page to the
queue."
So if it matters enough for reducing the time we hold the page table
lock, it surely adds "some" overhead in general.
You're the boss though, so if your experience tells you this is neccessary, then
I'm ok with that.
I did not do any measurements myself, I just did that intuitively as
above. After all, it's all pretty straight forward (keeping the existing
logic, we need a new one either way) and not that much code.
So unless there are strong opinions, I'd just leave the common case as
it was, and the odd case be special.
I think we can just reduce the code duplication easily:
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index d175c0f1e2c8..99b3e9408aa0 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -91,18 +91,21 @@ void tlb_flush_rmaps(struct mmu_gather *tlb, struct vm_area_struct *vma)
}
#endif
-static void tlb_batch_pages_flush(struct mmu_gather *tlb)
-{
- struct mmu_gather_batch *batch;
+/*
+ * We might end up freeing a lot of pages. Reschedule on a regular
+ * basis to avoid soft lockups in configurations without full
+ * preemption enabled. The magic number of 512 folios seems to work.
+ */
+#define MAX_NR_FOLIOS_PER_FREE 512
- for (batch = &tlb->local; batch && batch->nr; batch = batch->next) {
- struct encoded_page **pages = batch->encoded_pages;
+static void __tlb_batch_free_encoded_pages(struct mmu_gather_batch *batch)
+{
+ struct encoded_page **pages = batch->encoded_pages;
+ unsigned int nr, nr_pages;
- while (batch->nr) {
- /*
- * limit free batch count when PAGE_SIZE > 4K
- */
- unsigned int nr = min(512U, batch->nr);
+ while (batch->nr) {
+ if (!page_poisoning_enabled_static() && !want_init_on_free()) {
+ nr = min(MAX_NR_FOLIOS_PER_FREE, batch->nr);
/*
* Make sure we cover page + nr_pages, and don't leave
@@ -111,14 +114,39 @@ static void tlb_batch_pages_flush(struct mmu_gather *tlb)
if (unlikely(encoded_page_flags(pages[nr - 1]) &
ENCODED_PAGE_BIT_NR_PAGES_NEXT))
nr++;
+ } else {
+ /*
+ * With page poisoning and init_on_free, the time it
+ * takes to free memory grows proportionally with the
+ * actual memory size. Therefore, limit based on the
+ * actual memory size and not the number of involved
+ * folios.
+ */
+ for (nr = 0, nr_pages = 0;
+ nr < batch->nr && nr_pages < MAX_NR_FOLIOS_PER_FREE;
+ nr++) {
+ if (unlikely(encoded_page_flags(pages[nr]) &
+ ENCODED_PAGE_BIT_NR_PAGES_NEXT))
+ nr_pages += encoded_nr_pages(pages[++nr]);
+ else
+ nr_pages++;
+ }
+ }
- free_pages_and_swap_cache(pages, nr);
- pages += nr;
- batch->nr -= nr;
+ free_pages_and_swap_cache(pages, nr);
+ pages += nr;
+ batch->nr -= nr;
- cond_resched();
- }
+ cond_resched();
}
+}
+
+static void tlb_batch_pages_flush(struct mmu_gather *tlb)
+{
+ struct mmu_gather_batch *batch;
+
+ for (batch = &tlb->local; batch && batch->nr; batch = batch->next)
+ __tlb_batch_free_encoded_pages(batch);
tlb->active = &tlb->local;
}
--
2.43.0
--
Cheers,
David / dhildenb
