Re: [PATCH v3 0/8] make slab shrink lockless

[Qi Zheng]

On 2023/2/28 03:02, Roman Gushchin wrote:
> On Mon, Feb 27, 2023 at 09:31:51PM +0800, Qi Zheng wrote:
> > On 2023/2/27 03:51, Andrew Morton wrote:
> > > On Sun, 26 Feb 2023 22:46:47 +0800 Qi Zheng <zhengqi.arch@xxxxxxxxxxxxx> wrote:
> > Save the above script, then run test and touch commands.
> >
> > Then we can use the following perf command to view hotspots:
> >
> > perf top -U -F 999
> >
> > 1) Before applying this patchset:
> >
> >    32.31%  [kernel]           [k] down_read_trylock
> >    19.40%  [kernel]           [k] pv_native_safe_halt
> >    16.24%  [kernel]           [k] up_read
> >    15.70%  [kernel]           [k] shrink_slab
> >     4.69%  [kernel]           [k] _find_next_bit
> >     2.62%  [kernel]           [k] shrink_node
> >     1.78%  [kernel]           [k] shrink_lruvec
> >     0.76%  [kernel]           [k] do_shrink_slab
> >
> > 2) After applying this patchset:
> >
> >    27.83%  [kernel]           [k] _find_next_bit
> >    16.97%  [kernel]           [k] shrink_slab
> >    15.82%  [kernel]           [k] pv_native_safe_halt
> >     9.58%  [kernel]           [k] shrink_node
> >     8.31%  [kernel]           [k] shrink_lruvec
> >     5.64%  [kernel]           [k] do_shrink_slab
> >     3.88%  [kernel]           [k] mem_cgroup_iter
>
> Not opposing the intention of the patchset in any way (I actually think
> it's a good idea to make the shrinkers list lockless), but looking at
> both outputs above I think that the main problem is not the contention on
> the semaphore, but the reason of this contention.

Yes, in the above scenario, there is indeed no lock contention problem.

> It seems like often there is a long list of shrinkers which barely
> can reclaim any memory, but we're calling them again and again.
> In order to achieve real wins with real-life workloads, I guess
> it's what we should optimize.
>
> Thanks!

--
Thanks,
Qi