et>, Chris Zankel [off-list ref], Michal Simek [off-list ref], Thomas Bogendoerfer [off-list ref], linux-parisc@vger.kernel.org, Max Filippov [off-list ref], linux-kernel@vger.kernel.org, Dinh Nguyen [off-list ref], Palmer Dabbelt [off-list ref], Sven Schnelle [off-list ref], Guo Ren [off-list ref], Borislav Petkov [off-list ref], Johannes Berg [off-list ref], linuxppc-dev@lists.ozlabs.org, "David S . Miller" [off-list ref]
Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org
Sender: "Linuxppc-dev" [off-list ref]

On Fri, May 27, 2022 at 12:46:31PM +0200, Ingo Molnar wrote:

* Peter Xu [off-list ref] wrote:

quoted

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!

quoted

 arch/x86/mm/fault.c           |  4 ++++

Reviewed-by: Ingo Molnar <mingo@kernel.org>

Minor comment typo:

quoted

+		/*
+		 * 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.

quoted

 	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