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Tue, 17 Jun 2025 15:59:24 -0700 (PDT) Received: from localhost ([216.228.127.128]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-313c19d2ccdsm11283246a91.19.2025.06.17.15.59.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Jun 2025 15:59:24 -0700 (PDT) Date: Tue, 17 Jun 2025 18:59:22 -0400 From: Yury Norov To: Andrew Morton Cc: linux-kernel@vger.kernel.org, Rasmus Villemoes , John Stultz , Thomas Gleixner , Stephen Boyd Subject: Re: [PATCH v3 1/3] bitmap: generalize node_random() Message-ID: References: <20250617200854.60753-1-yury.norov@gmail.com> <20250617200854.60753-2-yury.norov@gmail.com> <20250617155056.5c1d292d8831e7c7a27c8e5f@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250617155056.5c1d292d8831e7c7a27c8e5f@linux-foundation.org> On Tue, Jun 17, 2025 at 03:50:56PM -0700, Andrew Morton wrote: > On Tue, 17 Jun 2025 16:08:51 -0400 Yury Norov wrote: > > > From: "Yury Norov [NVIDIA]" > > > > Generalize node_random() and make it available to general bitmaps and > > cpumasks users. > > > > Notice, find_first_bit() is generally faster than find_nth_bit(), and we > > employ it when there's a single set bit in the bitmap. > > > > See commit 3e061d924fe9c7b4 ("lib/nodemask: optimize node_random for > > nodemask with single NUMA node"). > > > > ... > > > > --- a/include/linux/nodemask.h > > +++ b/include/linux/nodemask.h > > @@ -492,21 +492,7 @@ static __always_inline int num_node_state(enum node_states state) > > static __always_inline int node_random(const nodemask_t *maskp) > > { > > #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1) > > - int w, bit; > > - > > - w = nodes_weight(*maskp); > > - switch (w) { > > - case 0: > > - bit = NUMA_NO_NODE; > > If the mask has no bits set, return -1. ... > If the mask has no bits set, return the mask's size. > > > + case 1: > > + /* Performance trick for single-bit bitmaps */ > > + return find_first_bit(addr, size); > > + default: > > + return find_nth_bit(addr, size, get_random_u32_below(w)); > > + } > > +} > > I'm not seeing how this is correct? Ahh... Indeed you're right. Thanks, Andrew, I'll send v4