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([2804:1b3:a800:b5ab:bd8c:63b4:4a03:edd]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-72dab812f21sm4161846b3a.43.2025.01.18.12.49.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 18 Jan 2025 12:49:48 -0800 (PST) Message-ID: <75e169509708600bc90db4cef897778929c4d4e2.camel@gmail.com> Subject: Atomic Limit Counter Add and Subtract, and QQ about goto From: Leonardo =?ISO-8859-1?Q?Br=E1s?= To: paulmck@kernel.org, perfbook@vger.kernel.org Date: Sat, 18 Jan 2025 17:49:43 -0300 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.54.3 Precedence: bulk X-Mailing-List: perfbook@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Hi Paul, I was reading the Counting Chapter in detail, and the QQ about avoiding a G= oto got my attention: Listing 5.13: Atomic Limit Counter Add and Subtract Quick Quiz 5.43: Yecch! Why the ugly goto on line 11 of Listing 5.13? Haven=E2=80=99t you heard of the break statement??? And I was thinking on an alternate solution to those, by calling slowpath a= s a function in this simple change: ``` inline int add_count_slowpath(unsigned long delta) { spin_lock(&gblcnt_mutex); globalize_count(); if (globalcountmax - globalcount - globalreserve < delta) { flush_local_count(); if (globalcountmax - globalcount - globalreserve < delta) { spin_unlock(&gblcnt_mutex); return 0; } } globalcount +=3D delta; balance_count(); spin_unlock(&gblcnt_mutex); return 1; } int add_count(unsigned long delta) { int c; int cm; int old; int new; do { split_counterandmax(&counterandmax, &old, &c, &cm); if (delta > MAX_COUNTERMAX || c + delta > cm) return add_count_slowpath(delta); new =3D merge_counterandmax(c + delta, cm);=09 while (atomic_cmpxchg(&counterandmax, old, new) !=3D old); return 1; } ``` Does it make sense? Does it introduce any overhead I couldn't see? Thanks! Leo