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To: Segher Boessenkool Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , nathanl@linux.ibm.com, arnd@arndb.de, tglx@linutronix.de, vincenzo.frascino@arm.com, luto@kernel.org, x86@kernel.org, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org References: <20200117085851.GS3191@gate.crashing.org> <3027b6d2-47a9-a871-7c52-050a5f9c6ab7@c-s.fr> <20200120151936.GB3191@gate.crashing.org> From: Christophe Leroy Message-ID: <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr> Date: Mon, 20 Jan 2020 18:08:23 +0100 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.9.1 MIME-Version: 1.0 In-Reply-To: <20200120151936.GB3191@gate.crashing.org> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: fr Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le 20/01/2020 à 16:19, Segher Boessenkool a écrit : > On Mon, Jan 20, 2020 at 02:56:00PM +0000, Christophe Leroy wrote: >>> Nice! Much better. >>> >>> It should be tested on more representative hardware, too, but this looks >>> promising alright :-) >> >> mpc832x (e300c2 core) at 333 MHz: >> >> Before: >> >> gettimeofday: vdso: 235 nsec/call >> clock-gettime-realtime: vdso: 244 nsec/call >> >> With the series: >> >> gettimeofday: vdso: 271 nsec/call >> clock-gettime-realtime: vdso: 281 nsec/call > > Those are important, and degrade ~15%. That is acceptable IMO, but do > you see a way to optimise this (later)? Not easy I think. First we have the unavoidable ASM entry function that can't be dropped because of the CR[SO] bit the set on error or clear on no error and that can't be done in C. In our ASM VDSO, fixed shifts are used, while in generic C VDSO, shifts are generic and read from the VDSO data. And there is still some funny code generated by GCC (8.1), like: 620: 7d 29 3c 30 srw r9,r9,r7 624: 21 87 00 20 subfic r12,r7,32 628: 7d 07 3c 31 srw. r7,r8,r7 62c: 7d 08 60 30 slw r8,r8,r12 630: 7d 0b 4b 78 or r11,r8,r9 634: 39 40 00 00 li r10,0 638: 40 82 00 84 bne 6bc <__c_kernel_clock_gettime+0x114> 63c: 81 23 00 24 lwz r9,36(r3) 640: 81 05 00 00 lwz r8,0(r5) ... 6bc: 7d 69 5b 78 mr r9,r11 6c0: 7c ea 3b 78 mr r10,r7 6c4: 7d 2b 4b 78 mr r11,r9 6c8: 4b ff ff 74 b 63c <__c_kernel_clock_gettime+0x94> This branch to 6bc is totally useless: - copying r11 into r9 is pointless as r9 is overwritten in 63c - copying back r9 into r11 is pointless as r11 has not been modified inbetween. - loading r10 with 0 then overwritting r10 with r7 when r7 is not 0 is pointless as well, could have directly put the result of srw. in r10. Christophe