linux-kernel - Re: [PATCH 1/3] lib/min_heap: Introduce non-inline versions of min heap API functions

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Message-ID: <20241014081358.GS17263@noisy.programming.kicks-ass.net>
Date: Mon, 14 Oct 2024 10:13:58 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: colyli@...e.de, kent.overstreet@...ux.dev, msakai@...hat.com,
	corbet@....net, mingo@...hat.com, acme@...nel.org,
	namhyung@...nel.org, akpm@...ux-foundation.org,
	mark.rutland@....com, alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, jserv@...s.ncku.edu.tw,
	linux-kernel@...r.kernel.org, linux-bcache@...r.kernel.org,
	dm-devel@...ts.linux.dev, linux-bcachefs@...r.kernel.org,
	linux-perf-users@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH 1/3] lib/min_heap: Introduce non-inline versions of min
 heap API functions

On Mon, Oct 14, 2024 at 02:47:01AM +0800, Kuan-Wei Chiu wrote:
> All current min heap API functions are marked with '__always_inline'.
> However, as the number of users increases, inlining these functions
> everywhere leads to a significant increase in kernel size.
> 
> In performance-critical paths, such as when perf events are enabled and
> min heap functions are called on every context switch, it is important
> to retain the inline versions for optimal performance. To balance this,
> the original inline functions are kept, and additional non-inline
> versions of the functions have been added in lib/min_heap.c.

The reason it is all __always_inline is because then the whole
min_heap_callbacks thing can be constant propagated and the func->less()
etc calls become direct calls.

Doing out of line for this stuff, makes them indirect calls, and
indirect calls are super retarded expensive ever since spectre. But also
things like kCFI add significant cost to indirect calls.

Something that would be a trivial subtract instruction becomes this
giant mess of an indirect function call.

Given the whole min_heap thing is basically a ton of less() and swp()
calls, I really don't think this trade off makes any kind of sense.