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[109.152.100.135]) by smtp.gmail.com with ESMTPSA id x13sm5736226wmi.20.2020.11.24.06.46.38 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 24 Nov 2020 06:46:38 -0800 (PST) To: John Garry , Jens Axboe , linux-block@vger.kernel.org, Omar Sandoval Cc: linux-kernel@vger.kernel.org References: <488177c02dccda60c5e8af2e53156c42b7f1acc0.1606058975.git.asml.silence@gmail.com> <17b6011c-b519-3332-e9b7-de36109db85a@huawei.com> From: Pavel Begunkov Autocrypt: addr=asml.silence@gmail.com; prefer-encrypt=mutual; keydata= mQINBFmKBOQBEAC76ZFxLAKpDw0bKQ8CEiYJRGn8MHTUhURL02/7n1t0HkKQx2K1fCXClbps bdwSHrhOWdW61pmfMbDYbTj6ZvGRvhoLWfGkzujB2wjNcbNTXIoOzJEGISHaPf6E2IQx1ik9 6uqVkK1OMb7qRvKH0i7HYP4WJzYbEWVyLiAxUj611mC9tgd73oqZ2pLYzGTqF2j6a/obaqha +hXuWTvpDQXqcOZJXIW43atprH03G1tQs7VwR21Q1eq6Yvy2ESLdc38EqCszBfQRMmKy+cfp W3U9Mb1w0L680pXrONcnlDBCN7/sghGeMHjGKfNANjPc+0hzz3rApPxpoE7HC1uRiwC4et83 CKnncH1l7zgeBT9Oa3qEiBlaa1ZCBqrA4dY+z5fWJYjMpwI1SNp37RtF8fKXbKQg+JuUjAa9 Y6oXeyEvDHMyJYMcinl6xCqCBAXPHnHmawkMMgjr3BBRzODmMr+CPVvnYe7BFYfoajzqzq+h EyXSl3aBf0IDPTqSUrhbmjj5OEOYgRW5p+mdYtY1cXeK8copmd+fd/eTkghok5li58AojCba jRjp7zVOLOjDlpxxiKhuFmpV4yWNh5JJaTbwCRSd04sCcDNlJj+TehTr+o1QiORzc2t+N5iJ NbILft19Izdn8U39T5oWiynqa1qCLgbuFtnYx1HlUq/HvAm+kwARAQABtDFQYXZlbCBCZWd1 bmtvdiAoc2lsZW5jZSkgPGFzbWwuc2lsZW5jZUBnbWFpbC5jb20+iQJOBBMBCAA4FiEE+6Ju PTjTbx479o3OWt5b1Glr+6UFAlmKBOQCGwMFCwkIBwIGFQgJCgsCBBYCAwECHgECF4AACgkQ Wt5b1Glr+6WxZA//QueaKHzgdnOikJ7NA/Vq8FmhRlwgtP0+E+w93kL+ZGLzS/cUCIjn2f4Q Mcutj2Neg0CcYPX3b2nJiKr5Vn0rjJ/suiaOa1h1KzyNTOmxnsqE5fmxOf6C6x+NKE18I5Jy xzLQoktbdDVA7JfB1itt6iWSNoOTVcvFyvfe5ggy6FSCcP+m1RlR58XxVLH+qlAvxxOeEr/e aQfUzrs7gqdSd9zQGEZo0jtuBiB7k98t9y0oC9Jz0PJdvaj1NZUgtXG9pEtww3LdeXP/TkFl HBSxVflzeoFaj4UAuy8+uve7ya/ECNCc8kk0VYaEjoVrzJcYdKP583iRhOLlZA6HEmn/+Gh9 4orG67HNiJlbFiW3whxGizWsrtFNLsSP1YrEReYk9j1SoUHHzsu+ZtNfKuHIhK0sU07G1OPN 2rDLlzUWR9Jc22INAkhVHOogOcc5ajMGhgWcBJMLCoi219HlX69LIDu3Y34uIg9QPZIC2jwr 24W0kxmK6avJr7+n4o8m6sOJvhlumSp5TSNhRiKvAHB1I2JB8Q1yZCIPzx+w1ALxuoWiCdwV M/azguU42R17IuBzK0S3hPjXpEi2sK/k4pEPnHVUv9Cu09HCNnd6BRfFGjo8M9kZvw360gC1 reeMdqGjwQ68o9x0R7NBRrtUOh48TDLXCANAg97wjPoy37dQE7e5Ag0EWYoE5AEQAMWS+aBV IJtCjwtfCOV98NamFpDEjBMrCAfLm7wZlmXy5I6o7nzzCxEw06P2rhzp1hIqkaab1kHySU7g dkpjmQ7Jjlrf6KdMP87mC/Hx4+zgVCkTQCKkIxNE76Ff3O9uTvkWCspSh9J0qPYyCaVta2D1 Sq5HZ8WFcap71iVO1f2/FEHKJNz/YTSOS/W7dxJdXl2eoj3gYX2UZNfoaVv8OXKaWslZlgqN jSg9wsTv1K73AnQKt4fFhscN9YFxhtgD/SQuOldE5Ws4UlJoaFX/yCoJL3ky2kC0WFngzwRF Yo6u/KON/o28yyP+alYRMBrN0Dm60FuVSIFafSqXoJTIjSZ6olbEoT0u17Rag8BxnxryMrgR dkccq272MaSS0eOC9K2rtvxzddohRFPcy/8bkX+t2iukTDz75KSTKO+chce62Xxdg62dpkZX xK+HeDCZ7gRNZvAbDETr6XI63hPKi891GeZqvqQVYR8e+V2725w+H1iv3THiB1tx4L2bXZDI DtMKQ5D2RvCHNdPNcZeldEoJwKoA60yg6tuUquvsLvfCwtrmVI2rL2djYxRfGNmFMrUDN1Xq F3xozA91q3iZd9OYi9G+M/OA01husBdcIzj1hu0aL+MGg4Gqk6XwjoSxVd4YT41kTU7Kk+/I 5/Nf+i88ULt6HanBYcY/+Daeo/XFABEBAAGJAjYEGAEIACAWIQT7om49ONNvHjv2jc5a3lvU aWv7pQUCWYoE5AIbDAAKCRBa3lvUaWv7pfmcEACKTRQ28b1y5ztKuLdLr79+T+LwZKHjX++P 4wKjEOECCcB6KCv3hP+J2GCXDOPZvdg/ZYZafqP68Yy8AZqkfa4qPYHmIdpODtRzZSL48kM8 LRzV8Rl7J3ItvzdBRxf4T/Zseu5U6ELiQdCUkPGsJcPIJkgPjO2ROG/ZtYa9DvnShNWPlp+R uPwPccEQPWO/NP4fJl2zwC6byjljZhW5kxYswGMLBwb5cDUZAisIukyAa8Xshdan6C2RZcNs rB3L7vsg/R8UCehxOH0C+NypG2GqjVejNZsc7bgV49EOVltS+GmGyY+moIzxsuLmT93rqyII 5rSbbcTLe6KBYcs24XEoo49Zm9oDA3jYvNpeYD8rDcnNbuZh9kTgBwFN41JHOPv0W2FEEWqe JsCwQdcOQ56rtezdCJUYmRAt3BsfjN3Jn3N6rpodi4Dkdli8HylM5iq4ooeb5VkQ7UZxbCWt UVMKkOCdFhutRmYp0mbv2e87IK4erwNHQRkHUkzbsuym8RVpAZbLzLPIYK/J3RTErL6Z99N2 m3J6pjwSJY/zNwuFPs9zGEnRO4g0BUbwGdbuvDzaq6/3OJLKohr5eLXNU3JkT+3HezydWm3W OPhauth7W0db74Qd49HXK0xe/aPrK+Cp+kU1HRactyNtF8jZQbhMCC8vMGukZtWaAwpjWiiH bA== Subject: Re: [PATCH v2 2/4] sbitmap: remove swap_lock Message-ID: <5f10de12-ffe1-dfef-d9ff-74b13a7f0426@gmail.com> Date: Tue, 24 Nov 2020 14:43:29 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <17b6011c-b519-3332-e9b7-de36109db85a@huawei.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 24/11/2020 14:22, John Garry wrote: > On 22/11/2020 15:35, Pavel Begunkov wrote: >> map->swap_lock protects map->cleared from concurrent modification, >> however sbitmap_deferred_clear() is already atomically drains it, so >> it's guaranteed to not loose bits on concurrent >> sbitmap_deferred_clear(). >> >> A one threaded tag heavy test on top of nullbk showed ~1.5% t-put >> increase, and 3% -> 1% cycle reduction of sbitmap_get() according to perf. >> >> Signed-off-by: Pavel Begunkov >> --- >>   include/linux/sbitmap.h |  5 ----- >>   lib/sbitmap.c           | 14 +++----------- >>   2 files changed, 3 insertions(+), 16 deletions(-) >> >> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h >> index e40d019c3d9d..74cc6384715e 100644 >> --- a/include/linux/sbitmap.h >> +++ b/include/linux/sbitmap.h >> @@ -32,11 +32,6 @@ struct sbitmap_word { >>        * @cleared: word holding cleared bits >>        */ >>       unsigned long cleared ____cacheline_aligned_in_smp; >> - >> -    /** >> -     * @swap_lock: Held while swapping word <-> cleared >> -     */ >> -    spinlock_t swap_lock; >>   } ____cacheline_aligned_in_smp; >>     /** >> diff --git a/lib/sbitmap.c b/lib/sbitmap.c >> index c1c8a4e69325..4fd877048ba8 100644 >> --- a/lib/sbitmap.c >> +++ b/lib/sbitmap.c >> @@ -15,13 +15,9 @@ >>   static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) >>   { >>       unsigned long mask, val; >> -    bool ret = false; >> -    unsigned long flags; >>   -    spin_lock_irqsave(&map->swap_lock, flags); >> - >> -    if (!map->cleared) >> -        goto out_unlock; >> +    if (!READ_ONCE(map->cleared)) >> +        return false; > > So if we race with another cpu, won't the 2nd cpu see that the mask is 0 returned from the xchg (not shown)? If so, it's odd to continue to do the CAS - or atomic not, from later patch - on a mask of 0. Yeah, but this part is legit and I don't expect it to be so contended to need an additional check, especially with atomic and from [3/4]. I'm more concerned about sbitmap_resize*() callers to do right synchronisation (e.g. quiesce) and not rely on that critical section I remove. Would be great if anyone can confirm that. > > Thanks, > John > >>         /* >>        * First get a stable cleared mask, setting the old mask to 0. >> @@ -35,10 +31,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap_word *map) >>           val = map->word; >>       } while (cmpxchg(&map->word, val, val & ~mask) != val); >>   -    ret = true; >> -out_unlock: >> -    spin_unlock_irqrestore(&map->swap_lock, flags); >> -    return ret; >> +    return true; >>   } >>     int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift, >> @@ -80,7 +73,6 @@ int sbitmap_init_node(struct sbitmap *sb, unsigned int depth, int shift, >>       for (i = 0; i < sb->map_nr; i++) { >>           sb->map[i].depth = min(depth, bits_per_word); >>           depth -= sb->map[i].depth; >> -        spin_lock_init(&sb->map[i].swap_lock); >>       } >>       return 0; >>   } >> > -- Pavel Begunkov