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Howlett" , Mark Brown , Michal Hocko , Kent Overstreet , Rick Edgecombe , linux-kernel@vger.kernel.org, llvm@lists.linux.dev References: <20241007053936.833392-1-nysal@linux.ibm.com> <20241029055133.121418-1-nysal@linux.ibm.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: <20241029055133.121418-1-nysal@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > 1b78d0: ld r9,2312(r13) # current > 1b78d4: ld r9,1888(r9) # current->mm > 1b78d8: cmpd cr7,r26,r9 > 1b78dc: beq cr7,1b7a70 > 1b78e0: hwsync > 1b78e4: cmplwi cr7,r27,128 > ....... > 1b7a70: lwz r9,176(r26) # atomic_read(&mm->membarrier_state) > 1b7a74: b 1b78e0 > > 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@0b02_0000d7e7_408 8 - -8 > e843419@01bb_000021d2_868 8 - -8 > finish_task_switch.isra 592 548 -44 > > Signed-off-by: Nysal Jan K.A. Thanks! Reviewed-by: Mathieu Desnoyers > --- > 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