On Fri, May 27, 2022 at 12:46:31PM +0200, Ingo Molnar wrote:
* Peter Xu <peterx@xxxxxxxxxx> wrote:
This patch provides a ~12% perf boost on my aarch64 test VM with a simple
program sequentially dirtying 400MB shmem file being mmap()ed and these are
the time it needs:
Before: 650.980 ms (+-1.94%)
After: 569.396 ms (+-1.38%)
Nice!
arch/x86/mm/fault.c | 4 ++++
Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
Minor comment typo:
+ /*
+ * We should do the same as VM_FAULT_RETRY, but let's not
+ * return -EBUSY since that's not reflecting the reality on
+ * what has happened - we've just fully completed a page
+ * fault, with the mmap lock released. Use -EAGAIN to show
+ * that we want to take the mmap lock _again_.
+ */
s/reflecting the reality on what has happened
/reflecting the reality of what has happened
Will fix.
ret = handle_mm_fault(vma, address, fault_flags, NULL);
+
+ if (ret & VM_FAULT_COMPLETED) {
+ /*
+ * NOTE: it's a pity that we need to retake the lock here
+ * to pair with the unlock() in the callers. Ideally we
+ * could tell the callers so they do not need to unlock.
+ */
+ mmap_read_lock(mm);
+ *unlocked = true;
+ return 0;
Indeed that's a pity - I guess more performance could be gained here,
especially in highly parallel threaded workloads?
Yes I think so.
The patch avoids the page fault retry, including the mmap lock/unlock side.
Now if we retake the lock for fixup_user_fault() we still safe time for
pgtable walks but the lock overhead will be somehow kept, just with smaller
critical sections.
Some fixup_user_fault() callers won't be affected as long as unlocked==NULL
is passed - e.g. the futex code path (fault_in_user_writeable). After all
they never needed to retake the lock before/after this patch.
It's about the other callers, and they may need some more touch-ups case by
case. Examples are follow_fault_pfn() in vfio and hva_to_pfn_remapped() in
KVM: both of them returns -EAGAIN when *unlocked==true. We need to teach
them to know "*unlocked==true" does not necessarily mean a retry attempt.
I think I can look into them if this patch can be accepted as a follow up.
Thanks for taking a look!
--
Peter Xu
