Re: [linus:master] [x86] 4817f70c25: stress-ng.mmapaddr.ops_per_sec 63.0% regression

[Qi Zheng]

On 2025/1/28 21:42, David Hildenbrand wrote:
> On 28.01.25 14:28, Peter Zijlstra wrote:
> > On Tue, Jan 28, 2025 at 12:39:51PM +0100, David Hildenbrand wrote:
> > > On 28.01.25 12:31, Peter Zijlstra wrote:
> >
> > > > > I recall a recent series to select MMU_GATHER_RCU_TABLE_FREE on x86
> > > > > unconditionally (@Peter, @Rik).
> > > >
> > > > Those changes should not have made it to Linus yet.
> > > >
> > > > /me updates git and checks...
> > > >
> > > > nope, nothing changed there ... yet
> > >
> > > Sorry, I wasn't quite clear. CONFIG_PT_RECLAIM made it upstream, 
> > > which has
> > > "select MMU_GATHER_RCU_TABLE_FREE" in kconfig.
> > >
> > > So I'm wondering if the degradation we see in this report is due to
> > > MMU_GATHER_RCU_TABLE_FREE being selected by CONFIG_PT_RECLAIM, and 
> > > we'd get
> > > the same result (degradation) when unconditionally enabling
> > > MMU_GATHER_RCU_TABLE_FREE.
> >
> > Ah, yes, put a RHEL based config (as is the case here) should already
> > have it selected due to PARAVIRT.
>
> Ah, right. Most distros will just have it enabled either way.
>
> But that would then mean that MMU_GATHER_RCU_TABLE_FREE is not the cause 

In addition, commit 718b13861d22 ("x86: mm: free page table pages by RCU 
instead of semi RCU")
also made a change, that is, when freeing a single page table pages, use
RCU to free the page table page instead of sending the IPI.

But in theory this should not cause performance regression, and I have
tested the performance of munmap with bpftrace and found no regression.

> for the regression here, and something else is going wrong.

It looks like it, I will investigate it carefully.

Thanks!

>