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[184.145.4.219]) by smtp.gmail.com with ESMTPSA id c141sm950456qke.12.2021.05.07.08.06.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 May 2021 08:06:01 -0700 (PDT) From: Peter Xu To: linux-mm@kvack.org, linux-kernel@vger.kernel.org Cc: Jan Kara , John Hubbard , peterx@redhat.com, Linus Torvalds , Michal Hocko , Kirill Tkhai , Kirill Shutemov , Oleg Nesterov , Andrew Morton , Jann Horn , Andrea Arcangeli , Jason Gunthorpe , Matthew Wilcox , Hugh Dickins Subject: [PATCH v2 2/3] mm: gup: allow FOLL_PIN to scale in SMP Date: Fri, 7 May 2021 11:05:52 -0400 Message-Id: <20210507150553.208763-3-peterx@redhat.com> X-Mailer: git-send-email 2.31.1 In-Reply-To: <20210507150553.208763-1-peterx@redhat.com> References: <20210507150553.208763-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Andrea Arcangeli has_pinned cannot be written by each pin-fast or it won't scale in SMP. This isn't "false sharing" strictly speaking (it's more like "true non-sharing"), but it creates the same SMP scalability bottleneck of "false sharing". To verify the improvement, below test is done on 40 cpus host with Intel(R) Xeon(R) CPU E5-2630 v4 @ 2.20GHz (must be with CONFIG_GUP_TEST=y): $ sudo chrt -f 1 ./gup_test -a -m 512 -j 40 Where we can get (average value for 40 threads): Old kernel: 477729.97 (+- 3.79%) New kernel: 89144.65 (+-11.76%) On a similar condition with 256 cpus, this commits increases the SMP scalability of pin_user_pages_fast() executed by different threads of the same process by more than 4000%. Signed-off-by: Andrea Arcangeli [peterx: rewrite commit message, add parentheses against "(A & B)"] Reviewed-by: John Hubbard Signed-off-by: Peter Xu --- mm/gup.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/mm/gup.c b/mm/gup.c index 63a079e361a3d..9933bc5c2eff2 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -1292,7 +1292,7 @@ static __always_inline long __get_user_pages_locked(struct mm_struct *mm, BUG_ON(*locked != 1); } - if (flags & FOLL_PIN) + if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned)) atomic_set(&mm->has_pinned, 1); /* @@ -2617,7 +2617,7 @@ static int internal_get_user_pages_fast(unsigned long start, FOLL_FAST_ONLY))) return -EINVAL; - if (gup_flags & FOLL_PIN) + if ((gup_flags & FOLL_PIN) && !atomic_read(¤t->mm->has_pinned)) atomic_set(¤t->mm->has_pinned, 1); if (!(gup_flags & FOLL_FAST_ONLY)) -- 2.31.1