On Thu, Mar 02, 2023 at 11:20:49AM -0500, Peter Xu wrote: > On Thu, Mar 02, 2023 at 11:47:35AM -0300, Marcelo Tosatti wrote: > > So it will be: > > > > #ifdef CONFIG_HAVE_CMPXCHG_LOCAL > > mod_zone_page_state > > inc_zone_page_state > > dec_zone_page_state > > mod_node_page_state > > inc_node_page_state > > dec_node_page_state > > __mod_zone_page_state (new function, calls mod_zone_page_state). > > __mod_node_page_state (new function, calls mod_node_page_state). > > __inc_zone_page_state > > __inc_node_page_state > > __dec_zone_page_state > > __dec_node_page_state > > #else > > __mod_zone_page_state (old, shared function for both CONFIG_HAVE_CMPXCHG_LOCAL and not) > > __mod_node_page_state > > __inc_zone_page_state > > __inc_node_page_state > > __dec_zone_page_state > > __dec_node_page_state > > mod_zone_page_state > > inc_zone_page_state > > dec_zone_page_state > > mod_node_page_state > > inc_node_page_state > > dec_node_page_state > > #endif > > > > Any suggestion on how to split this into multiple patchsets for easier > > reviewing? (can't think of anything obvious). > > I figured this out before saw this, but it did take me some time to read > carefully into the code base.. maybe it'll be a good idea to mention > something like above in the commit message to ease future reviewers (and > more likelyhood to attract the experts to start chim in)? > > One fundamental (but maybe another naive.. :) question on this code piece > (so not directly related to the changeset but maybe it is still..): > > AFAICT CONFIG_HAVE_CMPXCHG_LOCAL only means we can do cmpxchg() without > locking memory bus, CONFIG_HAVE_CMPXCHG_LOCAL means cmpxchg_local is implemented (that is cmpxchg which is atomic with respect to local CPU). LOCK cmpxchg is necessary for cmpxchg to be atomic on SMP. > however when !CONFIG_HAVE_CMPXCHG_LOCAL here we're not > using non-local version but using preempt_disable_nested() to make sure the > read is atomic. Does it really worth it? What happens if we use cmpxchg() > unconditionally, but just use local (e.g. no "LOCK" prefix) version when > CONFIG_HAVE_CMPXCHG_LOCAL? Can't use local version of cmpxchg because the vmstat counters are supposed to be accessed from different CPUs simultaneously (this is the objective of the patchset): CPU-0 CPU-1 vmstat_shepherd mod_zone_page_state xchg location LOCK cmpxchg location xchg locks memory bus implicitly. Is this what you are thinking about or did i misunderstood what you mean?
