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Tue, 25 Aug 2026 07:14:50 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Tue, 25 Aug 2026 16:14:48 +0200 To: JonasZhou-oc Cc: Andrew Morton , Uladzislau Rezki , linux-mm@kvack.org, linux-kernel@vger.kernel.org, jianhuizzzzz@gmail.com Subject: Re: [PATCH] mm/vmalloc: avoid false sharing with drain_vmap_work Message-ID: References: <20260825104659.100134-1-jonaszhou-oc@zhaoxin.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260825104659.100134-1-jonaszhou-oc@zhaoxin.com> On Tue, Aug 25, 2026 at 06:46:59PM +0800, JonasZhou-oc wrote: > free_vmap_area_noflush() queues drain_vmap_work after the number of > lazily freed pages exceeds lazy_max_pages(). Until the worker purges > those pages, concurrent frees keep calling schedule_work(). Even if > the work is already pending, queue_work_on() performs a locked > test_and_set_bit() on the pending bit in the work item. > > On the tested x86-64 build, drain_vmap_work and vmap_nodes occupy the > same 64-byte cache line. The work item starts at offset 0 and the > vmap_nodes pointer at offset 32. The latter is read by vmap allocation > and free paths, so updates to the work item invalidate a cache line > read by all CPUs. > > Put drain_vmap_work in the cacheline-aligned data section. Tests were > run on Linux 7.2. > > On a two-socket Intel Xeon Silver 4208 system using 16 workers, the > runtimes of vmalloc.fix_align, vmalloc.fix_size, and > vmalloc.no_block_alloc decreased by 12.61%, 5.78%, and 6.87%, > respectively. HITM samples for the affected cache line and total HITM > samples decreased by 96.55% and 13.36%, respectively. > > Signed-off-by: JonasZhou > --- > mm/vmalloc.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index f4fa227a8d7f..6b4b287e91f6 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -1088,7 +1088,12 @@ RB_DECLARE_CALLBACKS_MAX(static, free_vmap_area_rb_augment_cb, > static void reclaim_and_purge_vmap_areas(void); > static BLOCKING_NOTIFIER_HEAD(vmap_notify_list); > static void drain_vmap_area_work(struct work_struct *work); > -static DECLARE_WORK(drain_vmap_work, drain_vmap_area_work); > +/* > + * Keep the work item, whose pending bit is updated by freeing CPUs, > + * away from vmap metadata read by allocation and free paths. > + */ > +static __cacheline_aligned_in_smp > +DECLARE_WORK(drain_vmap_work, drain_vmap_area_work); > > static __cacheline_aligned_in_smp atomic_long_t vmap_lazy_nr; > > -- > 2.43.0 > > Makes sens to me. Reviewed-by: Uladzislau Rezki (Sony) -- Uladzislau Rezki