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

Dave Hansen
2024-10-22T19:53:04Z

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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.

[-- Attachment #2: 0001-x86-percpu-Avoid-comparing-unsigned-types-to-1.patch --]
[-- Type: text/x-patch, Size: 3180 bytes --]

From d63bcd350e1a3ba6196dadb26cb2f36f0ba1e182 Mon Sep 17 00:00:00 2001
From: Dave Hansen <dave.hansen@linux.intel.com>
Date: Fri, 18 Oct 2024 11:07:47 -0700
Subject: [PATCH] x86/percpu: Avoid comparing unsigned types to -1

clang warns when comparing an unsinged type to -1 since the comparison
is always false.

This can be quickly reproduced by setting CONFIG_WERROR=y and running:

make W=1 CC=clang-14 net/ipv4/tcp_output.o

net/ipv4/tcp_output.c:187:3: error: result of comparison of constant -1 with expression of type 'u8' (aka 'unsigned char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
187 | NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
188 | tp->compressed_ack);
| ~~~~~~~~~~~~~~~~~~~
...
arch/x86/include/asm/percpu.h:238:31: note: expanded from macro 'percpu_add_op'
238 | ((val) == 1 || (val) == -1)) ? \
| ~~~~~ ^ ~~

Fix this by avoiding a comparison of an uncast -1 to 'val'.

Doing this in addition to the existing 'pao_ID__' calculation would make it
even more unreadable. Remove 'pao_ID__' and replace it with the three
components of its calculation.

This preserves some unintuitive but useful behavior. For instance, gcc sees:

percpu_add_op(..., var, (u8)-1);

and can transform that into a "dec". Clang, on the other hand, sees the 'u8'
type and assumes that "(val) == -1" is false, which was the root of the
warning.

This is useful gcc behavior because:

#define this_cpu_sub(pcp, val) this_cpu_add(pcp, -(typeof(pcp))(val))

so any code that does:

this_cpu_sub(A, 1)

where 'A' is an unsigned type generates a "dec". Clang, on the other
hand generates a less-efficient "add".

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---
arch/x86/include/asm/percpu.h | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index c55a79d5feae..57d9759c692e 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -234,18 +234,19 @@ do { \
*/
#define percpu_add_op(size, qual, var, val) \
do { \
- const int pao_ID__ = (__builtin_constant_p(val) && \
- ((val) == 1 || (val) == -1)) ? \
- (int)(val) : 0; \
+ const int pao_const__ = __builtin_constant_p(val); \
+ const int pao_inc__ = (val) == 1; \
+ const int pao_dec__ = (typeof(var))(val) == \
+ (typeof(var))-1; \
\
if (0) { \
typeof(var) pao_tmp__; \
pao_tmp__ = (val); \
(void)pao_tmp__; \
} \
- if (pao_ID__ == 1) \
+ if (pao_const__ && pao_inc__) \
percpu_unary_op(size, qual, "inc", var); \
- else if (pao_ID__ == -1) \
+ else if (pao_const__ && pao_dec__) \
percpu_unary_op(size, qual, "dec", var); \
else \
percpu_binary_op(size, qual, "add", var, val); \
--
2.34.1