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

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Message-ID: <20250619075611.GX1613376@noisy.programming.kicks-ass.net>
Date: Thu, 19 Jun 2025 09:56:11 +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 Wed, Jun 18, 2025 at 11:33:59AM -0400, Steven Rostedt wrote:
> On Wed, 18 Jun 2025 16:13:45 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:

> > > +		info->cache = kzalloc(struct_size(cache, entries, UNWIND_MAX_ENTRIES),
> > > +				      GFP_KERNEL);  
> > 
> > And now you're one 'long' larger than a page. Surely that's a crap size
> > for an allocator?
> 
> Bah, Ingo suggested to put the counter in the allocation and I didn't think
> about the size going over the page. Good catch!
> 
> Since it can make one per task, it may be good to make this into a
> kmemcache.

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.