Re: [PATCH v2 08/11] mm/vmstat: switch counter modification to cmpxchg

[Peter Xu]

On Thu, Mar 02, 2023 at 04:11:32PM -0300, Marcelo Tosatti wrote:
> 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?

Yes, I think I wrongly interpreted cmpxchg_local() before on assuming it's
still atomic but an accelerated version of cmpxchg() that was only
supported on a few archs.  I double checked the code and spec on x86 - I
believe you're right.  Thanks,

-- 
Peter Xu