Messages in this thread | | | Date | Fri, 7 Jul 2017 10:31:28 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH v2 0/9] Remove spin_unlock_wait() |
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* Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Jul 06, 2017 at 09:20:24AM -0700, Paul E. McKenney wrote: > > On Thu, Jul 06, 2017 at 06:05:55PM +0200, Peter Zijlstra wrote: > > > On Thu, Jul 06, 2017 at 02:12:24PM +0000, David Laight wrote: > > > > From: Paul E. McKenney > > > > [ . . . ] > > > > > Now on the one hand I feel like Oleg that it would be a shame to loose > > > the optimization, OTOH this thing is really really tricky to use, > > > and has lead to a number of bugs already. > > > > I do agree, it is a bit sad to see these optimizations go. So, should > > this make mainline, I will be tagging the commits that spin_unlock_wait() > > so that they can be easily reverted should someone come up with good > > semantics and a compelling use case with compelling performance benefits. > > Ha!, but what would constitute 'good semantics' ? > > The current thing is something along the lines of: > > "Waits for the currently observed critical section > to complete with ACQUIRE ordering such that it will observe > whatever state was left by said critical section." > > With the 'obvious' benefit of limited interference on those actually > wanting to acquire the lock, and a shorter wait time on our side too, > since we only need to wait for completion of the current section, and > not for however many contender are before us.
There's another, probably just as significant advantage: queued_spin_unlock_wait() is 'read-only', while spin_lock()+spin_unlock() dirties the lock cache line. On any bigger system this should make a very measurable difference - if spin_unlock_wait() is ever used in a performance critical code path.
> Not sure I have an actual (micro) benchmark that shows a difference > though.
It should be pretty obvious from pretty much any profile, the actual lock+unlock sequence that modifies the lock cache line is essentially a global cacheline bounce.
> Is this all good enough to retain the thing, I dunno. Like I said, I'm > conflicted on the whole thing. On the one hand its a nice optimization, on the > other hand I don't want to have to keep fixing these bugs.
So on one hand it's _obvious_ that spin_unlock_wait() is both faster on the local _and_ the remote CPUs for any sort of use case where performance matters - I don't even understand how that can be argued otherwise.
The real question, does any use-case (we care about) exist.
Here's a quick list of all the use cases:
net/netfilter/nf_conntrack_core.c:
- This is I believe the 'original', historic spin_unlock_wait() usecase that still exists in the kernel. spin_unlock_wait() is only used in a rare case, when the netfilter hash is resized via nf_conntrack_hash_resize() - which is a very heavy operation to begin with. It will no doubt get slower with the proposed changes, but it probably does not matter. A networking person Acked-by would be nice though.
drivers/ata/libata-eh.c:
- Locking of the ATA port in ata_scsi_cmd_error_handler(), presumably this can race with IRQs and ioctls() on other CPUs. Very likely not performance sensitive in any fashion, on IO errors things stop for many seconds anyway.
ipc/sem.c:
- A rare race condition branch in the SysV IPC semaphore freeing code in exit_sem() - where even the main code flow is not performance sensitive, because typical database workloads get their semaphore arrays during startup and don't ever do heavy runtime allocation/freeing of them.
kernel/sched/completion.c:
- completion_done(). This is actually a (comparatively) rarely used completion API call - almost all the upstream usecases are in drivers, plus two in filesystems - neither usecase seems in a performance critical hot path. Completions typically involve scheduling and context switching, so in the worst case the proposed change adds overhead to a scheduling slow path.
So I'd argue that unless there's some surprising performance aspect of a completion_done() user, the proposed changes should not cause any performance trouble.
In fact I'd argue that any future high performance spin_unlock_wait() user is probably better off open coding the unlock-wait poll loop (and possibly thinking hard about eliminating it altogether). If such patterns pop up in the kernel we can think about consolidating them into a single read-only primitive again.
I.e. I think the proposed changes are doing no harm, and the unavailability of a generic primitive does not hinder future optimizations either in any significant fashion.
Thanks,
Ingo
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