linux-kernel - Re: [PATCH v10 05/14] unwind_user/deferred: Add unwind cache

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Message-ID: <20250619090436.GE1613200@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 11:04:36 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	bpf@...r.kernel.org, x86@...nel.org,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrii Nakryiko <andrii@...nel.org>,
	Indu Bhagat <indu.bhagat@...cle.com>,
	"Jose E. Marchesi" <jemarch@....org>,
	Beau Belgrave <beaub@...ux.microsoft.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v10 05/14] unwind_user/deferred: Add unwind cache

On Thu, Jun 19, 2025 at 04:47:14AM -0400, Steven Rostedt wrote:
> On Thu, 19 Jun 2025 09:56:11 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > Well, the trivial solution is to make it 511 and call it a day. Don't
> > make things complicated if you don't have to.
> 
> I don't know if this is more complicated, but it should make it fit
> nicely in a page:
> 
>   /* Make the cache fit in a page */
>   #define UNWIND_MAX_ENTRIES                                      \
>         ((PAGE_SIZE - sizeof(struct unwind_cache)) / sizeof(long))

Right, that's the fancy way of spelling 511 :-) Except on 32bit, where
it now spells 1023 instead.

Did you want that bitness difference?

Also, you ready for some *reaaally* big numbers on Power/ARM with 64K
pages? :-)