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[23.128.96.18]) by mx.google.com with ESMTP id c24si9892354eja.263.2020.09.07.09.30.44; Mon, 07 Sep 2020 09:31:06 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@linutronix.de header.s=2020 header.b=hjSqvsxT; dkim=neutral (no key) header.i=@linutronix.de header.s=2020e; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=linutronix.de Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730498AbgIGQ27 (ORCPT + 99 others); Mon, 7 Sep 2020 12:28:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33382 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730478AbgIGQ2u (ORCPT ); Mon, 7 Sep 2020 12:28:50 -0400 Received: from galois.linutronix.de (Galois.linutronix.de [IPv6:2a0a:51c0:0:12e:550::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 873E5C061573 for ; Mon, 7 Sep 2020 09:28:49 -0700 (PDT) Date: Mon, 7 Sep 2020 18:29:13 +0200 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1599496127; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=l3bmAbmLVMY/y/RdwYo9fB9Rf30FToictabiRw458dQ=; b=hjSqvsxTdzCKetdVFEOTAeupnenrRyt6LmI1mWmtgzulq4jMeTahceXU44+IHe/m0OU0EW lyI2opleqBsoP6ShJzGaqzqwZq4V6YtuyIQCci+uL9r+wwuSGeD88NneVNlwpIz39CHrDn V3tw3b6CGem2NZrGn1uX3R5U8hjuAChunyUPOuAr7erx0Gl8KHJWi/3mqrzGTvCV+S6diW T49Wbq46GPS/fut2fjAA8OAwz4iXtrvdk5YdKy8FgLNrq9MeCeeHq0UEHvR0P+ngfsQun2 6p+lkoVs2YxCJxLppSCXFePX+ydwn201WdYpSZ+22FvTEYdVOMDC/wj8NgResQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1599496127; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=l3bmAbmLVMY/y/RdwYo9fB9Rf30FToictabiRw458dQ=; b=l1QCcrlNaB9AF5AOEJnnzCFCl47j+J9WTq0FakrBTZoc5ernHjzhXlk6fmVtQ9P/pfPWgx ldn9sFrxtYB4KJAw== From: "Ahmed S. Darwish" To: peterz@infradead.org Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , "Sebastian A. Siewior" , LKML , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" Subject: Re: [PATCH v1 6/8] x86/tsc: Use seqcount_latch_t Message-ID: <20200907162913.GA178765@lx-t490> References: <20200827114044.11173-1-a.darwish@linutronix.de> <20200827114044.11173-7-a.darwish@linutronix.de> <20200904074142.GL2674@hirez.programming.kicks-ass.net> <20200904080312.GZ35926@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200904080312.GZ35926@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On Fri, Sep 04, 2020 at 10:03:12AM +0200, peterz@infradead.org wrote: > On Fri, Sep 04, 2020 at 09:41:42AM +0200, peterz@infradead.org wrote: > > On Thu, Aug 27, 2020 at 01:40:42PM +0200, Ahmed S. Darwish wrote: > > > > > __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data) > > > { > > > + seqcount_latch_t *seqcount; > > > int seq, idx; > > > > > > preempt_disable_notrace(); > > > > > > + seqcount = &this_cpu_ptr(&cyc2ns)->seq; > > > do { > > > - seq = this_cpu_read(cyc2ns.seq.sequence); > > > + seq = raw_read_seqcount_latch(seqcount); > > > idx = seq & 1; > > > > > > data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset); > > > data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul); > > > data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift); > > > > > > - } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence))); > > > + } while (read_seqcount_latch_retry(seqcount, seq)); > > > } > > > > So I worried about this change, it obviously generates worse code. But I > > was not expecting this: > > > > Before: > > > > 196: 0000000000000110 189 FUNC GLOBAL DEFAULT 1 native_sched_clock > > > > After: > > > > 195: 0000000000000110 399 FUNC GLOBAL DEFAULT 1 native_sched_clock > > > > That's _210_ bytes extra!! > > > > If you look at the disassembly of the thing after it's a complete > > trainwreck. > > The below delta fixes it again. > > --- > --- a/arch/x86/kernel/tsc.c > +++ b/arch/x86/kernel/tsc.c > @@ -68,21 +68,19 @@ early_param("tsc_early_khz", tsc_early_k > > __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data) > { > - seqcount_latch_t *seqcount; > int seq, idx; > > preempt_disable_notrace(); > > - seqcount = &this_cpu_ptr(&cyc2ns)->seq; > do { > - seq = raw_read_seqcount_latch(seqcount); > + seq = this_cpu_read(cyc2ns.seq.seqcount.sequence); > idx = seq & 1; > > data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset); > data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul); > data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift); > > - } while (read_seqcount_latch_retry(seqcount, seq)); > + } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence))); > } > > __always_inline void cyc2ns_read_end(void) I've been unsuccessful in reproducing this huge, 200+ bytes, difference. Can I please get the defconfig and GCC version? Here are the two competing implementations: noinline void cyc2ns_read_begin_v1(struct cyc2ns_data *data) { seqcount_latch_t *seqcount; int seq, idx; preempt_disable_notrace(); seqcount = &this_cpu_ptr(&cyc2ns)->seq; do { seq = raw_read_seqcount_latch(seqcount); idx = seq & 1; data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset); data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul); data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift); } while (read_seqcount_latch_retry(seqcount, seq)); } noinline void cyc2ns_read_begin_v2(struct cyc2ns_data *data) { int seq, idx; preempt_disable_notrace(); do { seq = this_cpu_read(cyc2ns.seq.seqcount.sequence); idx = seq & 1; data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset); data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul); data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift); } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence))); } And here are their resulting gcc-7/8/4.9 binary output (DEBUG_PREEMPT=y, but setting DEBUG_KERNEL/DEBUG_PREEMPT=n also doesn't show any big _v1 vs. _v2 difference; similarly when using gcc-10): gcc-7 ===== ffffffff81028890 : ffffffff81028890: 65 ff 05 a9 e6 fe 7e incl %gs:0x7efee6a9(%rip) # 16f40 <__preempt_count> ffffffff81028893: R_X86_64_PC32 __preempt_count-0x4 ffffffff81028897: 48 c7 c6 40 a5 1f 00 mov $0x1fa540,%rsi ffffffff8102889a: R_X86_64_32S .data..percpu+0x1fa540 ffffffff8102889e: 48 89 f2 mov %rsi,%rdx ffffffff810288a1: 65 48 03 15 bf 8a fe add %gs:0x7efe8abf(%rip),%rdx # 11368 ffffffff810288a8: 7e ffffffff810288a5: R_X86_64_PC32 this_cpu_off-0x4 ffffffff810288a9: 8b 4a 20 mov 0x20(%rdx),%ecx ffffffff810288ac: 89 c8 mov %ecx,%eax ffffffff810288ae: 83 e0 01 and $0x1,%eax ffffffff810288b1: 48 c1 e0 04 shl $0x4,%rax ffffffff810288b5: 48 01 f0 add %rsi,%rax ffffffff810288b8: 65 4c 8b 40 08 mov %gs:0x8(%rax),%r8 ffffffff810288bd: 4c 89 47 08 mov %r8,0x8(%rdi) ffffffff810288c1: 65 44 8b 00 mov %gs:(%rax),%r8d ffffffff810288c5: 44 89 07 mov %r8d,(%rdi) ffffffff810288c8: 65 8b 40 04 mov %gs:0x4(%rax),%eax ffffffff810288cc: 89 47 04 mov %eax,0x4(%rdi) ffffffff810288cf: 8b 42 20 mov 0x20(%rdx),%eax ffffffff810288d2: 39 c8 cmp %ecx,%eax ffffffff810288d4: 75 d3 jne ffffffff810288a9 ffffffff810288d6: f3 c3 repz retq ffffffff810288d8: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) ffffffff810288df: 00 ffffffff810288e0 : ffffffff810288e0: 65 ff 05 59 e6 fe 7e incl %gs:0x7efee659(%rip) # 16f40 <__preempt_count> ffffffff810288e3: R_X86_64_PC32 __preempt_count-0x4 ffffffff810288e7: 48 c7 c1 40 a5 1f 00 mov $0x1fa540,%rcx ffffffff810288ea: R_X86_64_32S .data..percpu+0x1fa540 ffffffff810288ee: 48 89 c8 mov %rcx,%rax ffffffff810288f1: 65 48 03 05 6f 8a fe add %gs:0x7efe8a6f(%rip),%rax # 11368 ffffffff810288f8: 7e ffffffff810288f5: R_X86_64_PC32 this_cpu_off-0x4 ffffffff810288f9: 65 8b 15 60 1c 1d 7f mov %gs:0x7f1d1c60(%rip),%edx # 1fa560 ffffffff810288fc: R_X86_64_PC32 .data..percpu+0x1fa55c ffffffff81028900: 89 d0 mov %edx,%eax ffffffff81028902: 83 e0 01 and $0x1,%eax ffffffff81028905: 48 c1 e0 04 shl $0x4,%rax ffffffff81028909: 48 01 c8 add %rcx,%rax ffffffff8102890c: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi ffffffff81028911: 48 89 77 08 mov %rsi,0x8(%rdi) ffffffff81028915: 65 8b 30 mov %gs:(%rax),%esi ffffffff81028918: 89 37 mov %esi,(%rdi) ffffffff8102891a: 65 8b 40 04 mov %gs:0x4(%rax),%eax ffffffff8102891e: 89 47 04 mov %eax,0x4(%rdi) ffffffff81028921: 65 8b 05 38 1c 1d 7f mov %gs:0x7f1d1c38(%rip),%eax # 1fa560 ffffffff81028924: R_X86_64_PC32 .data..percpu+0x1fa55c ffffffff81028928: 39 c2 cmp %eax,%edx ffffffff8102892a: 75 cd jne ffffffff810288f9 ffffffff8102892c: f3 c3 repz retq ffffffff8102892e: 66 90 xchg %ax,%ax gcc-8 ===== ffffffff810281a0 : ffffffff810281a0: 65 ff 05 99 ed fe 7e incl %gs:0x7efeed99(%rip) # 16f40 <__preempt_count> ffffffff810281a3: R_X86_64_PC32 __preempt_count-0x4 ffffffff810281a7: 49 c7 c0 40 a5 1f 00 mov $0x1fa540,%r8 ffffffff810281aa: R_X86_64_32S .data..percpu+0x1fa540 ffffffff810281ae: 4c 89 c1 mov %r8,%rcx ffffffff810281b1: 65 48 03 0d af 91 fe add %gs:0x7efe91af(%rip),%rcx # 11368 ffffffff810281b8: 7e ffffffff810281b5: R_X86_64_PC32 this_cpu_off-0x4 ffffffff810281b9: 8b 51 20 mov 0x20(%rcx),%edx ffffffff810281bc: 89 d0 mov %edx,%eax ffffffff810281be: 83 e0 01 and $0x1,%eax ffffffff810281c1: 48 c1 e0 04 shl $0x4,%rax ffffffff810281c5: 4c 01 c0 add %r8,%rax ffffffff810281c8: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi ffffffff810281cd: 48 89 77 08 mov %rsi,0x8(%rdi) ffffffff810281d1: 65 8b 30 mov %gs:(%rax),%esi ffffffff810281d4: 89 37 mov %esi,(%rdi) ffffffff810281d6: 65 8b 40 04 mov %gs:0x4(%rax),%eax ffffffff810281da: 89 47 04 mov %eax,0x4(%rdi) ffffffff810281dd: 8b 41 20 mov 0x20(%rcx),%eax ffffffff810281e0: 39 d0 cmp %edx,%eax ffffffff810281e2: 75 d5 jne ffffffff810281b9 ffffffff810281e4: c3 retq ffffffff810281e5: 66 66 2e 0f 1f 84 00 data16 nopw %cs:0x0(%rax,%rax,1) ffffffff810281ec: 00 00 00 00 ffffffff810281f0 : ffffffff810281f0: 65 ff 05 49 ed fe 7e incl %gs:0x7efeed49(%rip) # 16f40 <__preempt_count> ffffffff810281f3: R_X86_64_PC32 __preempt_count-0x4 ffffffff810281f7: 48 c7 c1 40 a5 1f 00 mov $0x1fa540,%rcx ffffffff810281fa: R_X86_64_32S .data..percpu+0x1fa540 ffffffff810281fe: 48 89 c8 mov %rcx,%rax ffffffff81028201: 65 48 03 05 5f 91 fe add %gs:0x7efe915f(%rip),%rax # 11368 ffffffff81028208: 7e ffffffff81028205: R_X86_64_PC32 this_cpu_off-0x4 ffffffff81028209: 65 8b 15 50 23 1d 7f mov %gs:0x7f1d2350(%rip),%edx # 1fa560 ffffffff8102820c: R_X86_64_PC32 .data..percpu+0x1fa55c ffffffff81028210: 89 d0 mov %edx,%eax ffffffff81028212: 83 e0 01 and $0x1,%eax ffffffff81028215: 48 c1 e0 04 shl $0x4,%rax ffffffff81028219: 48 01 c8 add %rcx,%rax ffffffff8102821c: 65 48 8b 70 08 mov %gs:0x8(%rax),%rsi ffffffff81028221: 48 89 77 08 mov %rsi,0x8(%rdi) ffffffff81028225: 65 8b 30 mov %gs:(%rax),%esi ffffffff81028228: 89 37 mov %esi,(%rdi) ffffffff8102822a: 65 8b 40 04 mov %gs:0x4(%rax),%eax ffffffff8102822e: 89 47 04 mov %eax,0x4(%rdi) ffffffff81028231: 65 8b 05 28 23 1d 7f mov %gs:0x7f1d2328(%rip),%eax # 1fa560 ffffffff81028234: R_X86_64_PC32 .data..percpu+0x1fa55c ffffffff81028238: 39 c2 cmp %eax,%edx ffffffff8102823a: 75 cd jne ffffffff81028209 ffffffff8102823c: c3 retq ffffffff8102823d: 0f 1f 00 nopl (%rax) gcc-4.9 ======= ffffffff810ab900 : ffffffff810ab900: 55 push %rbp ffffffff810ab901: 48 89 fd mov %rdi,%rbp ffffffff810ab904: 53 push %rbx ffffffff810ab905: 65 ff 05 f4 b6 f6 7e incl %gs:0x7ef6b6f4(%rip) # 17000 <__preempt_count> ffffffff810ab908: R_X86_64_PC32 __preempt_count-0x4 ffffffff810ab90c: e8 df da c9 00 callq ffffffff81d493f0 ffffffff810ab90d: R_X86_64_PC32 debug_smp_processor_id-0x4 ffffffff810ab911: 89 c0 mov %eax,%eax ffffffff810ab913: 48 c7 c3 00 ab 02 00 mov $0x2ab00,%rbx ffffffff810ab916: R_X86_64_32S .data..percpu+0x2ab00 ffffffff810ab91a: 48 03 1c c5 80 87 65 add -0x7d9a7880(,%rax,8),%rbx ffffffff810ab921: 82 ffffffff810ab91e: R_X86_64_32S __per_cpu_offset ffffffff810ab922: 8b 53 20 mov 0x20(%rbx),%edx ffffffff810ab925: 89 d0 mov %edx,%eax ffffffff810ab927: 83 e0 01 and $0x1,%eax ffffffff810ab92a: 48 c1 e0 04 shl $0x4,%rax ffffffff810ab92e: 65 48 8b 88 08 ab 02 mov %gs:0x2ab08(%rax),%rcx ffffffff810ab935: 00 ffffffff810ab932: R_X86_64_32S .data..percpu+0x2ab08 ffffffff810ab936: 48 89 4d 08 mov %rcx,0x8(%rbp) ffffffff810ab93a: 65 8b 88 00 ab 02 00 mov %gs:0x2ab00(%rax),%ecx ffffffff810ab93d: R_X86_64_32S .data..percpu+0x2ab00 ffffffff810ab941: 89 4d 00 mov %ecx,0x0(%rbp) ffffffff810ab944: 65 8b 80 04 ab 02 00 mov %gs:0x2ab04(%rax),%eax ffffffff810ab947: R_X86_64_32S .data..percpu+0x2ab04 ffffffff810ab94b: 89 45 04 mov %eax,0x4(%rbp) ffffffff810ab94e: 8b 43 20 mov 0x20(%rbx),%eax ffffffff810ab951: 39 c2 cmp %eax,%edx ffffffff810ab953: 75 cd jne ffffffff810ab922 ffffffff810ab955: 5b pop %rbx ffffffff810ab956: 5d pop %rbp ffffffff810ab957: c3 retq ffffffff810ab958: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1) ffffffff810ab95f: 00 ffffffff810ab960 : ffffffff810ab960: 65 ff 05 99 b6 f6 7e incl %gs:0x7ef6b699(%rip) # 17000 <__preempt_count> ffffffff810ab963: R_X86_64_PC32 __preempt_count-0x4 ffffffff810ab967: 65 8b 15 b2 f1 f7 7e mov %gs:0x7ef7f1b2(%rip),%edx # 2ab20 ffffffff810ab96a: R_X86_64_PC32 .data..percpu+0x2ab1c ffffffff810ab96e: 89 d0 mov %edx,%eax ffffffff810ab970: 83 e0 01 and $0x1,%eax ffffffff810ab973: 48 c1 e0 04 shl $0x4,%rax ffffffff810ab977: 65 48 8b 88 08 ab 02 mov %gs:0x2ab08(%rax),%rcx ffffffff810ab97e: 00 ffffffff810ab97b: R_X86_64_32S .data..percpu+0x2ab08 ffffffff810ab97f: 48 89 4f 08 mov %rcx,0x8(%rdi) ffffffff810ab983: 65 8b 88 00 ab 02 00 mov %gs:0x2ab00(%rax),%ecx ffffffff810ab986: R_X86_64_32S .data..percpu+0x2ab00 ffffffff810ab98a: 89 0f mov %ecx,(%rdi) ffffffff810ab98c: 65 8b 80 04 ab 02 00 mov %gs:0x2ab04(%rax),%eax ffffffff810ab98f: R_X86_64_32S .data..percpu+0x2ab04 ffffffff810ab993: 89 47 04 mov %eax,0x4(%rdi) ffffffff810ab996: 65 8b 05 83 f1 f7 7e mov %gs:0x7ef7f183(%rip),%eax # 2ab20 ffffffff810ab999: R_X86_64_PC32 .data..percpu+0x2ab1c ffffffff810ab99d: 39 c2 cmp %eax,%edx ffffffff810ab99f: 75 c6 jne ffffffff810ab967 ffffffff810ab9a1: f3 c3 repz retq ffffffff810ab9a3: 66 66 66 66 2e 0f 1f data16 data16 data16 nopw %cs:0x0(%rax,%rax,1) ffffffff810ab9aa: 84 00 00 00 00 00 Thanks, -- Ahmed S. Darwish Linutronix GmbH