From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range Date: Tue, 21 Apr 2020 10:30:43 +0200 Message-ID: <20200421083043.GP20730@hirez.programming.kicks-ass.net> References: <20200403090048.938-1-yezhenyu2@huawei.com> <20200403090048.938-7-yezhenyu2@huawei.com> <20200420121055.GF20696@hirez.programming.kicks-ass.net> <20200420200616.44c7c7ea@oasis.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20200420200616.44c7c7ea@oasis.local.home> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane-mx.org@lists.infradead.org To: Steven Rostedt Cc: mark.rutland@arm.com, catalin.marinas@arm.com, linux-mm@kvack.org, guohanjun@huawei.com, will@kernel.org, linux-arch@vger.kernel.org, yuzhao@google.com, maz@kernel.org, suzuki.poulose@arm.com, steven.price@arm.com, arm@kernel.org, Dave.Martin@arm.com, arnd@arndb.de, Zhenyu Ye , npiggin@gmail.com, zhangshaokun@hisilicon.com, broonie@kernel.org, xiexiangyou@huawei.com, prime.zeng@hisilicon.com, kuhn.chenqun@huawei.com, tglx@linutronix.de, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org List-Id: linux-arch.vger.kernel.org On Mon, Apr 20, 2020 at 08:06:16PM -0400, Steven Rostedt wrote: > Peter Zijlstra wrote: > > On Fri, Apr 03, 2020 at 05:00:48PM +0800, Zhenyu Ye wrote: > > > +static inline int tlb_get_level(struct mmu_gather *tlb) > > > +{ > > > + int sum = tlb->cleared_ptes + tlb->cleared_pmds + > > > + tlb->cleared_puds + tlb->cleared_p4ds; > > > + > > > + if (sum != 1) > > > + return 0; > > > + else if (tlb->cleared_ptes) > > > + return 3; > > > + else if (tlb->cleared_pmds) > > > + return 2; > > > + else if (tlb->cleared_puds) > > > + return 1; > > > + > > > + return 0; > > > +} > > > > That's some mighty wonky code. Please look at the generated asm. > > Without even looking at the generated asm, if a condition returns, > there's no reason to add an else for that condition. Not really the point; he wants to guarantee he only returns >0 when there's a single bit set. But the thing is, cleared_* is a bitfield, and I'm afraid that the above will result in some terrible code-gen. Maybe something like: if (tlb->cleared_ptes && !(tlb->cleared_pmds || tlb->cleared_puds || tlb->cleared_p4ds)) return 3; if (tlb->cleared_pmds && !(tlb->cleared_ptes || tlb->cleared_puds || tlb->cleared_p4ds)) return 2; if (tlb->cleared_puds && !(tlb->cleared_ptes || tlb->cleared_pmds || tlb->cleared_p4ds)) return 1; return 0; Which I admit is far too much typing, but I suspect it generates far saner code (just a few masks and branches). But maybe the compiler surprises us, what do I konw. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 21 Apr 2020 10:30:43 +0200 From: Peter Zijlstra Subject: Re: [PATCH v1 6/6] arm64: tlb: Set the TTL field in flush_tlb_range Message-ID: <20200421083043.GP20730@hirez.programming.kicks-ass.net> References: <20200403090048.938-1-yezhenyu2@huawei.com> <20200403090048.938-7-yezhenyu2@huawei.com> <20200420121055.GF20696@hirez.programming.kicks-ass.net> <20200420200616.44c7c7ea@oasis.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200420200616.44c7c7ea@oasis.local.home> Sender: owner-linux-mm@kvack.org To: Steven Rostedt Cc: Zhenyu Ye , mark.rutland@arm.com, will@kernel.org, catalin.marinas@arm.com, aneesh.kumar@linux.ibm.com, akpm@linux-foundation.org, npiggin@gmail.com, arnd@arndb.de, maz@kernel.org, suzuki.poulose@arm.com, tglx@linutronix.de, yuzhao@google.com, Dave.Martin@arm.com, steven.price@arm.com, broonie@kernel.org, guohanjun@huawei.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, arm@kernel.org, xiexiangyou@huawei.com, prime.zeng@hisilicon.com, zhangshaokun@hisilicon.com, kuhn.chenqun@huawei.com List-ID: Message-ID: <20200421083043.-cr5MZyo8aysaeXy_Naa9Y8HaDeiWc1IWPYJgZMYBQc@z> On Mon, Apr 20, 2020 at 08:06:16PM -0400, Steven Rostedt wrote: > Peter Zijlstra wrote: > > On Fri, Apr 03, 2020 at 05:00:48PM +0800, Zhenyu Ye wrote: > > > +static inline int tlb_get_level(struct mmu_gather *tlb) > > > +{ > > > + int sum = tlb->cleared_ptes + tlb->cleared_pmds + > > > + tlb->cleared_puds + tlb->cleared_p4ds; > > > + > > > + if (sum != 1) > > > + return 0; > > > + else if (tlb->cleared_ptes) > > > + return 3; > > > + else if (tlb->cleared_pmds) > > > + return 2; > > > + else if (tlb->cleared_puds) > > > + return 1; > > > + > > > + return 0; > > > +} > > > > That's some mighty wonky code. Please look at the generated asm. > > Without even looking at the generated asm, if a condition returns, > there's no reason to add an else for that condition. Not really the point; he wants to guarantee he only returns >0 when there's a single bit set. But the thing is, cleared_* is a bitfield, and I'm afraid that the above will result in some terrible code-gen. Maybe something like: if (tlb->cleared_ptes && !(tlb->cleared_pmds || tlb->cleared_puds || tlb->cleared_p4ds)) return 3; if (tlb->cleared_pmds && !(tlb->cleared_ptes || tlb->cleared_puds || tlb->cleared_p4ds)) return 2; if (tlb->cleared_puds && !(tlb->cleared_ptes || tlb->cleared_pmds || tlb->cleared_p4ds)) return 1; return 0; Which I admit is far too much typing, but I suspect it generates far saner code (just a few masks and branches). But maybe the compiler surprises us, what do I konw.