linux-kernel - Re: [PATCH v2] sched/membarrier: Fix redundant load of membarrier

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6f394b76-3db4-4a7b-9fdd-737c6812e7ef@efficios.com>
Date: Tue, 29 Oct 2024 13:51:02 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: "Nysal Jan K.A." <nysal@...ux.ibm.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Michael Ellerman <mpe@...erman.id.au>
Cc: Segher Boessenkool <segher@...nel.crashing.org>,
 Stephen Rothwell <sfr@...b.auug.org.au>,
 Peter Zijlstra <peterz@...radead.org>, linuxppc-dev@...ts.ozlabs.org,
 Nathan Chancellor <nathan@...nel.org>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Vlastimil Babka <vbabka@...e.cz>, "Liam R. Howlett"
 <Liam.Howlett@...cle.com>, Mark Brown <broonie@...nel.org>,
 Michal Hocko <mhocko@...e.com>, Kent Overstreet <kent.overstreet@...ux.dev>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>, linux-kernel@...r.kernel.org,
 llvm@...ts.linux.dev
Subject: Re: [PATCH v2] sched/membarrier: Fix redundant load of
 membarrier_state

On 2024-10-29 01:51, Nysal Jan K.A. wrote:
> On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> is not selected, sync_core_before_usermode() is a no-op.
> In membarrier_mm_sync_core_before_usermode() the compiler does not
> eliminate redundant branches and load of mm->membarrier_state
> for this case as the atomic_read() cannot be optimized away.
> 
> Here's a snippet of the code generated for finish_task_switch() on powerpc
> prior to this change:
> 
> 1b786c:   ld      r26,2624(r30)   # mm = rq->prev_mm;
> .......
> 1b78c8:   cmpdi   cr7,r26,0
> 1b78cc:   beq     cr7,1b78e4 <finish_task_switch+0xd0>
> 1b78d0:   ld      r9,2312(r13)    # current
> 1b78d4:   ld      r9,1888(r9)     # current->mm
> 1b78d8:   cmpd    cr7,r26,r9
> 1b78dc:   beq     cr7,1b7a70 <finish_task_switch+0x25c>
> 1b78e0:   hwsync
> 1b78e4:   cmplwi  cr7,r27,128
> .......
> 1b7a70:   lwz     r9,176(r26)     # atomic_read(&mm->membarrier_state)
> 1b7a74:   b       1b78e0 <finish_task_switch+0xcc>
> 
> This was found while analyzing "perf c2c" reports on kernels prior
> to commit c1753fd02a00 ("mm: move mm_count into its own cache line")
> where mm_count was false sharing with membarrier_state.
> 
> There is a minor improvement in the size of finish_task_switch().
> The following are results from bloat-o-meter for ppc64le:
> 
> GCC 7.5.0
> ---------
> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
> Function                                     old     new   delta
> finish_task_switch                           884     852     -32
> 
> GCC 12.2.1
> ----------
> add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
> Function                                     old     new   delta
> finish_task_switch.isra                      852     820     -32
> 
> LLVM 17.0.6
> -----------
> add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-36 (-36)
> Function                                     old     new   delta
> rt_mutex_schedule                            120     104     -16
> finish_task_switch                           792     772     -20
> 
> Results on aarch64:
> 
> GCC 14.1.1
> ----------
> add/remove: 0/2 grow/shrink: 1/1 up/down: 4/-60 (-56)
> Function                                     old     new   delta
> get_nohz_timer_target                        352     356      +4
> e843419@...2_0000d7e7_408                      8       -      -8
> e843419@...b_000021d2_868                      8       -      -8
> finish_task_switch.isra                      592     548     -44
> 
> Signed-off-by: Nysal Jan K.A. <nysal@...ux.ibm.com>

Thanks!

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> ---
> V1 -> V2:
> - Add results for aarch64
> - Add a comment describing the changes
> ---
>   include/linux/sched/mm.h | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 928a626725e6..b13474825130 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -531,6 +531,13 @@ enum {
>   
>   static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
>   {
> +	/*
> +	 * The atomic_read() below prevents CSE. The following should
> +	 * help the compiler generate more efficient code on architectures
> +	 * where sync_core_before_usermode() is a no-op.
> +	 */
> +	if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
> +		return;
>   	if (current->mm != mm)
>   		return;
>   	if (likely(!(atomic_read(&mm->membarrier_state) &

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com