Re: [PATCH 00/14] Reduce preallocations for maple tree

[Hugh Dickins]

On Tue, 6 Jun 2023, Yin, Fengwei wrote:
> On 6/6/2023 10:41 AM, Hugh Dickins wrote:
> > On Mon, 5 Jun 2023, Liam R. Howlett wrote:
> >>
> >> You mean "mm: update validate_mm() to use vma iterator" here I guess.  I
> >> have it as a different commit id in my branch.
> >>
> >> I 'restored' some of the checking because I was able to work around not
> >> having the mt_dump() definition with the vma iterator.  I'm now
> >> wondering how wide spread CONFIG_DEBUG_VM is used and if I should not
> >> have added these extra checks.
> > 
> > Most CONFIG_DEBUG_VM checks are quite cheap, mostly VM_BUG_ONs for
> Indeed. I had CONFIG_DEBUG_VM enabled and didn't see surprise perf report.
> 
> 
> > easily checked conditions.  If validate_mm() is still the kind of thing
> > it used to be, checking through every vma on every mmap operation, please
> > don't bring that into CONFIG_DEBUG_VM - it distorts performance too much,
> > so always used to be under a separate CONFIG_DEBUG_VM_RB instead.
> So does this mean CONFIG_DEBUG_VM is allowed to be enabled for performance
> testing? Thanks.

I was going to say:
No, I did not mean that: I just meant that even developers not doing
strict performance testing still like to keep a rough eye on performance
changes; and historically CONFIG_DEBUG_VM has not distorted very much.

But then I wonder about certain distros which (wrongly or rightly) turn
CONFIG_DEBUG_VM on: I expect they do performance testing on their kernels.

Hugh