On Wed, Dec 13, 2023 at 7:01 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > On Wed, Dec 13, 2023 at 01:04:14PM +0000, Yosry Ahmed wrote: > > memcg_kmem_uncharge_page() is an inline wrapper around > > __memcg_kmem_uncharge_page() that checks memcg_kmem_online() before > > making the function call. Internally, __memcg_kmem_uncharge_page() has a > > folio_memcg_kmem() check. > > > > The only direct user of __memcg_kmem_uncharge_page(), > > free_pages_prepare(), checks PageMemcgKmem() before calling it to avoid > > the function call if possible. Move the folio_memcg_kmem() check from > > __memcg_kmem_uncharge_page() to memcg_kmem_uncharge_page() as > > PageMemcgKmem() -- which does the same thing under the hood. Now > > free_pages_prepare() can also use memcg_kmem_uncharge_page(). > > I think you've just pessimised all the other places which call > memcg_kmem_uncharge_page(). It's a matter of probabilities. In > free_pages_prepare(), most of the pages being freed are not accounted > to memcg. Whereas in fork() we are absolutely certain that the pages > were accounted because we accounted them. The check was already there for other callers, but it was inside __memcg_kmem_uncharge_page(). IIUC, the only change for other callers is an extra call to compound_head(), and they are not hot paths AFAICT so it shouldn't be noticeable. Am I missing something? Perhaps your point is about how branch prediction works across function call boundaries? or is this not about performance at all? > > I think this is a bad change.
