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([2605:a601:adae:4500:b86c:e734:b34:45c6]) by smtp.gmail.com with ESMTPSA id cv8-20020a056638498800b0046b4f448cc5sm7933260jab.91.2024.01.04.07.33.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 04 Jan 2024 07:33:02 -0800 (PST) Message-ID: <236f584e-76bf-4d98-8db4-562b627558bb@sifive.com> Date: Thu, 4 Jan 2024 09:33:01 -0600 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 04/12] riscv: Only send remote fences when some other CPU is online Content-Language: en-US To: Alexandre Ghiti Cc: Palmer Dabbelt , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <20240102220134.3229156-1-samuel.holland@sifive.com> <20240102220134.3229156-5-samuel.holland@sifive.com> From: Samuel Holland In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Stat-Signature: o6pjw8dqc9cdygqmjreex9skrftb39wm X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: A02D1180020 X-HE-Tag: 1704382383-71234 X-HE-Meta: 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 zNO/j5UG g4oej5Bhp7C4UNAKhyWjj24hNNVFoeFeOMpN4R81oZOt/4rGO/9hP+xgH6Jq/T+4WQNp3dto+2ubU0cMb95f26zNm9p8w43TCD6nDPkeHfv/CNxVkvsPa+86ZtXBnfczVJ2Qt1hQO1HEeFncmiKBVEVnNOcY7JVuid5ZV0B2+6sTPmUH20gk9riVzQjBviLfXiJmB0sliJVJhyl26sM9nl2o9QVE9YlDjfHFiAyAHmEzORDAIFdAt+f2kDq5+YeNr4rs411230qit99fpdcLhTObQWGq5bqCmA3UgKU4L5DUfyHIAdy80oZTTNxS6CK2KEQSxV41uU2MiH79TeuFdvFdLhfL5rbMUbX6vS85HGga+cFOK6GDVZ81XJpVADZvntUGBu5jScW2agMu34YZ8SMFNqZIaqmgaK52mY0LLW2jY42Eq9J+UP9bpUrI3odj1WZsdekolZHwCjYg= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000035, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Alex, On 2024-01-04 6:33 AM, Alexandre Ghiti wrote: > On Tue, Jan 2, 2024 at 11:01 PM Samuel Holland > wrote: >> >> If no other CPU is online, a local cache or TLB flush is sufficient. >> These checks can be constant-folded when SMP is disabled. >> >> Signed-off-by: Samuel Holland >> --- >> >> Changes in v4: >> - New patch for v4 >> >> arch/riscv/mm/cacheflush.c | 4 +++- >> arch/riscv/mm/tlbflush.c | 4 +++- >> 2 files changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c >> index 47c485bc7df0..f7933ae88a55 100644 >> --- a/arch/riscv/mm/cacheflush.c >> +++ b/arch/riscv/mm/cacheflush.c >> @@ -21,7 +21,9 @@ void flush_icache_all(void) >> { >> local_flush_icache_all(); >> >> - if (riscv_use_sbi_for_rfence()) >> + if (num_online_cpus() < 2) >> + return; >> + else if (riscv_use_sbi_for_rfence()) >> sbi_remote_fence_i(NULL); >> else >> on_each_cpu(ipi_remote_fence_i, NULL, 1); >> diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c >> index 2f18fe6fc4f3..37b3c93e3c30 100644 >> --- a/arch/riscv/mm/tlbflush.c >> +++ b/arch/riscv/mm/tlbflush.c >> @@ -73,7 +73,9 @@ static void __ipi_flush_tlb_all(void *info) >> >> void flush_tlb_all(void) >> { >> - if (riscv_use_sbi_for_rfence()) >> + if (num_online_cpus() < 2) >> + local_flush_tlb_all(); >> + else if (riscv_use_sbi_for_rfence()) >> sbi_remote_sfence_vma_asid(NULL, 0, FLUSH_TLB_MAX_SIZE, FLUSH_TLB_NO_ASID); >> else >> on_each_cpu(__ipi_flush_tlb_all, NULL, 1); >> -- >> 2.42.0 >> > > on_each_cpu() already deals correctly with a single online cpu, the > only thing to optimize here is the SBI rfence. So I'd move this new > test in sbi_remote_sfence_vma_asid() and sbi_remote_fence_i() to avoid > the superfluous M-mode entry when only one cpu is online by checking > the cpumask. And since sbi_remote_fence_i() is used in another What specific cpumask check are you suggesting? In sbi_remote_sfence_vma_asid() I don't think we can assume the local cpu is always included in the cpumask (which we _can_ assume here), so it would need to construct/compare the whole bitmap. That's much more expensive than the atomic load here. > function (flush_icache_mm()), we could also take advantage of this > optimization when only the local cpu must be flushed. flush_icache_mm() already has a "local" variable which it uses to skip the call to sbi_remote_fence_i(). Same with the broadcast check in __flush_tlb_range(). So no additional check is needed there. Those two functions, plus the two changed in this patch, are the only call sites of the sbi_*() functions. I think it makes more sense to optimize the four call sites than adding an additional check in the sbi_*() functions. Regards, Samuel