Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0% regression

[Peter Zijlstra]

On Tue, May 10, 2022 at 11:05:01AM -0700, Linus Torvalds wrote:

> I think from a pure lock standpoint, it's the right thing to do (no
> unnecessary bouncing, with the lock releaser doing just one write, and
> the head waiter spinning on it is doing the right thing).
> 
> But I think this is an example of where you end up having that
> spinning on the lock possibly then being a disturbance on the other
> fields around the lock.
> 
> I wonder if Waiman / PeterZ / Will have any comments on that. Maybe
> that "spin on the lock itself" is just fundamentally the only correct
> thing, but since my initial reaction was "no, we're spinning on the
> mcs node", maybe that would be _possible_?
> 
> We do have a lot of those spinlocks embedded in other data structures
> cases. And if "somebody else is waiting for the lock" contends badly
> with "the lock holder is doing a lot of writes close to the lock",
> then that's not great.

The immediate problem is that we don't always have a node. Notably we
only do the whole MCS queueing thing when there's more than 1 contender.

Always doing the MCS thing had a hefty performance penalty vs the
simpler spinlock implementations for the uncontended and light contended
lock cases (by far the most common scenario) due to the extra cache-miss
of getting an MCS node.