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Mon, 09 Sep 2019 04:18:14 -0700 (PDT) Received: by uranus.localdomain (Postfix, from userid 1000) id 0CE07460667; Mon, 9 Sep 2019 14:18:12 +0300 (MSK) Date: Mon, 9 Sep 2019 14:18:12 +0300 From: Cyrill Gorcunov To: Dmitry Safonov Cc: linux-kernel@vger.kernel.org, Dmitry Safonov <0x7f454c46@gmail.com>, Adrian Reber , Alexander Viro , Andrei Vagin , Andy Lutomirski , Ingo Molnar , Oleg Nesterov , Pavel Emelyanov , Thomas Gleixner , containers@lists.linux-foundation.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH 4/9] select: Micro-optimise __estimate_accuracy() Message-ID: <20190909111812.GB1508@uranus> References: <20190909102340.8592-1-dima@arista.com> <20190909102340.8592-5-dima@arista.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190909102340.8592-5-dima@arista.com> User-Agent: Mutt/1.12.0 (2019-05-25) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Mon, Sep 09, 2019 at 11:23:35AM +0100, Dmitry Safonov wrote: > Shift on s64 is faster than division, use it instead. > > As the result of the patch there is a hardly user-visible effect: > poll(), select(), etc syscalls will be a bit more precise on ~2.3% > than before because 1000 != 1024 :) > > Signed-off-by: Dmitry Safonov > --- > fs/select.c | 9 ++++----- > 1 file changed, 4 insertions(+), 5 deletions(-) > > diff --git a/fs/select.c b/fs/select.c > index 12cdefd3be2d..2477c202631e 100644 > --- a/fs/select.c > +++ b/fs/select.c > @@ -51,15 +51,14 @@ > > static long __estimate_accuracy(ktime_t slack) > { > - int divfactor = 1000; > - > if (slack < 0) > return 0; > > - if (task_nice(current) > 0) > - divfactor = divfactor / 5; > + /* A bit more precise than 0.1% */ > + slack = slack >> 10; > > - slack = ktime_divns(slack, divfactor); > + if (task_nice(current) > 0) > + slack = slack * 5; > > if (slack > MAX_SLACK) > return MAX_SLACK; Compiler precompute constants so it doesn't do division here. But I didn't read the series yet so I might be missing something obvious.