On 6/6/2023 11:08 AM, Hugh Dickins wrote: > 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. Fair enough. Thanks for explanation. Regards Yin, Fengwei > > Hugh
