linux-kernel - Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when comparing in percpu_add

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Message-ID: <c22fd9c5-6727-46c2-a811-784315edf7cb@intel.com>
Date: Tue, 22 Oct 2024 12:53:01 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Ingo Molnar <mingo@...nel.org>, Uros Bizjak <ubizjak@...il.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
 Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
 Christoph Lameter <cl@...ux.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>
Subject: Re: [PATCH v1 1/1] x86/percpu: Cast -1 to argument type when
 comparing in percpu_add_op()

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:

> 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.

View attachment "0001-x86-percpu-Avoid-comparing-unsigned-types-to-1.patch" of type "text/x-patch" (3180 bytes)