Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when comparing in percpu_add_op()

[Andy Shevchenko]

On Tue, Oct 22, 2024 at 12:53:01PM -0700, Dave Hansen wrote:
> On 10/17/24 11:18, Peter Zijlstra wrote:
> > On Wed, Oct 16, 2024 at 12:44:18PM -0700, Dave Hansen wrote:

...

> >> Would anybody hate if we broke this up a bit, like:
> >>
> >>         const typeof(var) _val = val;
> >>         const int paoconst = __builtin_constant_p(val);
> >>         const int paoinc   = paoconst && ((_val) == 1);
> >>         const int paodec   = paoconst && ((_val) == (typeof(var))-1);
> >>
> >> and then did
> >>
> >> 	if (paoinc)
> >> 		percpu_unary_op(size, qual, "inc", var);
> >> 	...
> > I think that is an overall improvement. Proceed! 🙂
> 
> I poked at this a bit:

Thanks for looking into this!

> > https://git.kernel.org/pub/scm/linux/kernel/git/daveh/devel.git/commit/?h=testme&id=30e0899c6ab7fe1134e4b96db963f0be89b1dd5a
> 
> I believe it functions fine.  But it surprised me with a few things.
> Here's one.  I assumed that doing an add((unsigned)-1) would be rare.
> It's not.  It's actually pretty common because this:
> 
> #define this_cpu_sub(pcp, val)  this_cpu_add(pcp, -(typeof(pcp))(val))
> 
> ends up causing problems when 'pcp' is an unsigned type.  For example,
> in this chain:
> 
> 	mem_cgroup_exit ->
> 	obj_cgroup_put ->
> 	percpu_ref_put ->
> 	percpu_ref_put_many(ref, 1) ->
> 	this_cpu_sub
> 
> the compiler can see the '1' constant.  It effectively expands to:
> 
> 	this_cpu_add(pcp, -(unsigned long)(1))
> 
> With the old code, gcc manages to generate a 'dec'.  Clang generates an
> 'add'.  With my hack above both compilers generate an 'add'.  This
> actually matters in some code that seems potentially rather performance
> sensitive:
> 
> add/remove: 0/0 grow/shrink: 219/9 up/down: 755/-141 (614)
> Function                                     old     new   delta
> flush_end_io                                 905    1070    +165
> x86_pmu_cancel_txn                           242     338     +96
> lru_add                                      554     594     +40
> mlock_folio_batch                           3264    3300     +36
> compaction_alloc                            3813    3838     +25
> tcp_leave_memory_pressure                     86     110     +24
> account_guest_time                           270     287     +17
> ...
> 
> So I think Peter's version was the best.  It shuts up clang and also
> preserves the existing (good) gcc 'sub' behavior.  I'll send it out for
> real in a bit, but I'm thinking of something like the attached patch.

I am fine as long as you keep the (added) test cases and maybe even extend
them. I dunno how you will go with the fact that Andrew applied my version
already.

...

> This can be quickly reproduced by setting CONFIG_WERROR=y and running:
> 
> 	make W=1 CC=clang-14 net/ipv4/tcp_output.o

Hint: You can use LLVM=-14 instead of CC=clang-14.

-- 
With Best Regards,
Andy Shevchenko