On Tue, Jun 16, 2020 at 11:55:17AM -0700, Linus Torvalds wrote: > On Mon, Jun 15, 2020 at 3:16 PM Peter Xu <pet...@redhat.com> wrote: > > > > This series tries to address all of them by introducing > > mm_fault_accounting() > > first, so that we move all the page fault accounting into the common code > > base, > > then call it properly from arch pf handlers just like handle_mm_fault(). > > Hmm. > > So having looked at this a bit more, I'd actually like to go even > further, and just get rid of the per-architecture code _entirely_. > > Here's a straw-man patch to the generic code - the idea is mostly laid > out in the comment that I'm just quoting here directly too: > > /* > * Do accounting in the common code, to avoid unnecessary > * architecture differences or duplicated code. > * > * We arbitrarily make the rules be: > * > * - faults that never even got here (because the address > * wasn't valid). That includes arch_vma_access_permitted() > * failing above. > * > * So this is expressly not a "this many hardware page > * faults" counter. Use the hw profiling for that. > * > * - incomplete faults (ie RETRY) do not count (see above). > * They will only count once completed. > * > * - the fault counts as a "major" fault when the final > * successful fault is VM_FAULT_MAJOR, or if it was a > * retry (which implies that we couldn't handle it > * immediately previously). > * > * - if the fault is done for GUP, regs wil be NULL and > * no accounting will be done (but you _could_ pass in > * your own regs and it would be accounted to the thread > * doing the fault, not to the target!) > */ > > the code itself in the patch is > > (a) pretty trivial and self-evident > > (b) INCOMPLETE > > that (b) is worth noting: this patch won't compile on its own. It > intentionally leaves all the users without the new 'regs' argument, > because you obviously simply need to remove all the code that > currently tries to do any accounting. > > Comments?
Looks clean to me. The definition of "major faults" will slightly change even for those who has done the "major |= fault & MAJOR" operations before, but at least I can't see anything bad about that either... To make things easier, we can use the 1st patch to introduce this change, however pass regs==NULL at the callers to never trigger this accounting. Then we can still use one patch for each arch to do the final convertions. > > This is a bigger change, but I think it might be worth it to _really_ > consolidate the major/minor logic. > > One detail worth noting: I do wonder if we should put the > > perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, addr); > > just in the arch code at the top of the fault handling, and consider > it entirely unrelated to the major/minor fault handling. The > major/minor faults fundamnetally are about successes. But the plain > PERF_COUNT_SW_PAGE_FAULTS could be about things that fail, including > things that never even get to this point at all. > > I'm not convinced it's useful to have three SW events that are defined > to be A=B+C. IMHO it's still common to have a "total" statistics in softwares even if each of the subsets are accounted separately. Here it's just a bit special because there're only two elements so the addition is so straightforward. It seems a trade-off on whether we'd like to do the accounting of errornous faults, or we want to make it cleaner by put them altogether but only successful page faults. I slightly preferred the latter due to the fact that I failed to find great usefulness out of keeping error fault accountings, but no strong opinions.. Thanks, -- Peter Xu
