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Peter Anvin" , Andi Kleen Subject: Re: [PATCH v2 2/2] perf: Don't throttle based on NMI watchdog events Message-ID: References: <33d87384aa5f96af76949d1399476779dd4f4fce.1777483573.git.calvin@wbinvd.org> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wednesday 04/29 at 15:15 -0700, Ian Rogers wrote: > On Wed, Apr 29, 2026 at 3:09 PM Calvin Owens wrote: > > On Wednesday 04/29 at 20:38 +0000, sashiko-bot@kernel.org wrote: > > > Additionally, is it possible for user-defined PMU events with expensive > > > overhead (like heavy eBPF programs attached to slow tracepoints) to exploit > > > this hardcoded frequency bypass? > > > > No, I don't think ebpf programs run in NMI context? > > Unfortunately Sashiko isn't currently reading replied emails, but you > could copy-paste this feedback into an AI. I know it doesn't reply. I was hoping for input from somebody who knew, more automated guessing from more LLMs is not really helpful at all :) > BPF programs can run in the NMI context, which means they can > introduce delays. I'm not familiar with all the side-effects of and > restrictions on this. Thanks for the clarification. Thinking about it more: it seems to me that if they run from NMIs and their runtime is unbounded, this is just a consequence of that and not a new problem. If they are somehow bounded, then this must be bounded in the same way. Either way, I don't think it's a real problem as far as this patch goes. But maybe I'm not being imaginative enough. Thanks, Calvin