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Date: Wed, 22 Jan 2025 15:16:16 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: x86@kernel.org, Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	linux-kernel@vger.kernel.org, Indu Bhagat <indu.bhagat@oracle.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
	Ian Rogers <irogers@google.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	linux-perf-users@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	linux-toolchains@vger.kernel.org, Jordan Rome <jordalgo@meta.com>,
	Sam James <sam@gentoo.org>, linux-trace-kernel@vger.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>,
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	Andy Lutomirski <luto@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Weinan Liu <wnliu@google.com>
Subject: Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding
 interface
Message-ID: <20250122141616.GU7145@noisy.programming.kicks-ass.net>
References: <cover.1737511963.git.jpoimboe@kernel.org>
 <6052e8487746603bdb29b65f4033e739092d9925.1737511963.git.jpoimboe@kernel.org>
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In-Reply-To: <20250122133730.GQ7145@noisy.programming.kicks-ass.net>

On Wed, Jan 22, 2025 at 02:37:30PM +0100, Peter Zijlstra wrote:
> On Tue, Jan 21, 2025 at 06:31:20PM -0800, Josh Poimboeuf wrote:
> > +/*
> > + * The context cookie is a unique identifier which allows post-processing to
> > + * correlate kernel trace(s) with user unwinds.  The high 12 bits are the CPU
> 
> s/12/16/ ?
> 
> > + * id; the lower 48 bits are a per-CPU entry counter.
> > + */
> > +static u64 ctx_to_cookie(u64 cpu, u64 ctx)
> > +{
> > +	BUILD_BUG_ON(NR_CPUS > 65535);
> > +	return (ctx & ((1UL << 48) - 1)) | (cpu << 48);
> > +}

Also, I have to note that 0 is a valid return value here, which will
give a ton of fun.