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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43fe4e3a341sm57552815f8f.24.2026.04.23.05.24.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Apr 2026 05:24:47 -0700 (PDT) Date: Thu, 23 Apr 2026 13:24:46 +0100 From: David Laight To: Mathias Stearn Cc: Thomas Gleixner , Dmitry Vyukov , Jinjie Ruan , linux-man@vger.kernel.org, Mark Rutland , Mathieu Desnoyers , Catalin Marinas , Will Deacon , Boqun Feng , "Paul E. McKenney" , Chris Kennelly , regressions@lists.linux.dev, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Peter Zijlstra , Ingo Molnar , Blake Oler Subject: Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere Message-ID: <20260423132446.70478a78@pumpkin> In-Reply-To: References: <87zf2u28d1.ffs@tglx> <87wlxy22x7.ffs@tglx> <87ik9i0xlj.ffs@tglx> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: regressions@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Thu, 23 Apr 2026 12:51:22 +0200 Mathias Stearn wrote: > On Thu, Apr 23, 2026 at 12:39=E2=80=AFPM Thomas Gleixner wrote: > > The kernel clears rseq_cs reliably when user space was interrupted and: > > > > the task was preempted > > or > > the return from interrupt delivers a signal > > > > If the task invoked a syscall then there is absolutely no reason to do > > either of this because syscalls from within a critical section are a > > bug and catched when enabling rseq debugging. > > > > The original code did this along with unconditionally updating CPU/MMCID > > which resulted in ~15% performance regression on a syscall heavy > > database benchmark once glibc started to register rseq. =20 >=20 > Just to be clear TCMalloc does not need either rseq_cs to be cleared > or cpu_id_start to be written to on syscalls because it doesn't do > syscalls from critical sections. It will actually benefit (slightly) > from not updating cpu_id_start on syscalls. >=20 > It is specifically in the cases where an rseq would need to be aborted > (preemption, signals, migration, and membarrier IPI with the rseq > flag) that TCMalloc relies on cpu_id_start being written. It does rely > on that write even when not inside the critical section, because it > effectively uses that to detect if there were any would-cause-abort > events in between two critical sections. But since it leaves the > rseq_cs pointer non-null between critical sections, so you dont need > to add _any_ overhead for programs that never make use of rseq after > registration, or add any overhead to syscalls even for those who do. >=20 That sounds like one long rseq sequence where the 'restart' path detects that some of the operations have already been done. David