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b=G6PPtaLkflZk3MTvspaS+cIYDyds2Bql7vWVSowsj9PNZLKEOHqY20F3VBht+OmDG Lry/15CZ5gFe31X80DSUoQ1tpbI6z83yCLGwfnfZCQxS5pQV68fqYELjtOli9J45nf OxhlIinkSrsqdgmJLwHJ9RWFWygnrGjIaCGfOwZKQdsL2tUJ2dNMcuQDNa95GKg5TY EmxDYSNOpuLe+L7Xk69DS6kjVF9HMLzkTVoMc4emBCGXglXqr8FwnPQFqZQzOezhLs mAnoM4uDW2LgiLudE6yHax2WzwLAAUnuf1Tp/UmZ9veCFtrjI98umQl8YsX3aYF7AE 4HcVId5PTEnfA== Date: Thu, 20 Aug 2026 19:27:16 -0600 (MDT) From: Paul Walmsley To: Xie Bo cc: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , linux-riscv@lists.infradead.org, Dennis Zhou , Tejun Heo , Christoph Lameter , Jonathan Corbet , Shuah Khan , linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] riscv: Add native this_cpu_cmpxchg() support In-Reply-To: <20260730094407.16588-1-xb@ultrarisc.com> Message-ID: <8b8cbb4a-474d-1de4-eaf2-dd8b4e6ca8ec@kernel.org> References: <20260730094407.16588-1-xb@ultrarisc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Stat-Signature: dadjgszo564xmyan9cbym1p6jk5g8wno X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 47BC140006 X-HE-Tag: 1787275641-328710 X-HE-Meta: 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 pXi6EUo3 2wGnAMMBtWFTxEl0rPoGTgHhmvfXDA0Ua8nfG1S7H1RhRHo7xqfJVhgKtlvrBLspZdFvb7kFNBBmLxOAQeWih/BUaq61rG7GhTNixOej1fzpflgJyRkfqzzp4ynaAGre7dfN6MFVkUO7kaBkf1BNsFqm6r7kV31LHBfhcYHfkqfQl2IrMZCscn7UsaWZvkefOyuiNwsj5r1XBwy7vnJQtjlDfxdHajVDIoke5hnF5CPLLG1UTpTW+ZT94PV1CEYJ1CVWjJGMQRzjlWnH7S3AgV73+4Uk9iF9rnloBdTNweCQzxp1VOMaYN3fVabxsAv5EzkH41vd8+2W/MZOXTja7BnltFv/X5FHRaN96ICGzZa7EXno= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi, On Thu, 30 Jul 2026, Xie Bo wrote: > RISC-V falls back to the generic this_cpu_cmpxchg() implementation, > which serializes the operation by disabling local interrupts. > Consequently, HAVE_CMPXCHG_LOCAL is unset and users such as > percpu_counter cannot use their cmpxchg-based fast paths. > > Implement the 4-byte this_cpu_cmpxchg() operation with cmpxchg_local(), > and provide the 8-byte operation on RV64. Pin execution while resolving > the current CPU pointer so the LR/SC loop operates on one per-CPU > instance, while allowing interrupt-context updates to race through the > atomic operation. > > Copy the old and new values to private temporaries before invoking > cmpxchg_local(). This avoids collisions between local variable names in > nested statement-expression macros. > > The 1- and 2-byte operations continue to use the generic fallback. > > Select HAVE_CMPXCHG_LOCAL and update the architecture feature matrix. > > A percpu_counter_add() benchmark using the default batch value reduced > the median time per operation by 77.3% (10 runs of 5,000,000 > operations). > > In three 60-second stress-ng fork runs, median throughput increased by > 4.6%. A 120-second combined fork and VM stress test completed without > rss-counter errors or validation failures. > > Signed-off-by: Xie Bo This patch seems to be missing 128-bit cmpxchg support. Care to add it? thanks, - Paul