linux-kernel - Re: [PATCH v2 0/6] Improve visibility of writeback

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Message-ID: <ZgXFrabAqunDctVp@slm.duckdns.org>
Date: Thu, 28 Mar 2024 09:31:57 -1000
From: Tejun Heo <tj@...nel.org>
To: Kent Overstreet <kent.overstreet@...ux.dev>
Cc: Kemeng Shi <shikemeng@...weicloud.com>, akpm@...ux-foundation.org,
	willy@...radead.org, jack@...e.cz, bfoster@...hat.com,
	dsterba@...e.com, mjguzik@...il.com, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 0/6] Improve visibility of writeback

Hello, Kent.

On Thu, Mar 28, 2024 at 03:24:35PM -0400, Kent Overstreet wrote:
> fs/bcachefs/time_stats.c has some code that's going to be moving out to
> lib/ at some point, after I switch it to MAD; if you could hook that up
> as well to a few points we could see at a glance if there are stalls
> happening in the writeback path.

Using BPF (whether through bcc or bpftrace) is likely a better approach for
this sort of detailed instrumentation. Fixed debug information is useful and
it's also a common occurrence that they don't quite reveal the full picture
of what one's trying to understand and one needs to dig a bit deeper, wider,
aggregate data in a different way, or whatever.

So, rather than adding more fixed infrastructure, I'd suggest adding places
which can easily be instrumented using the existing tools (they are really
great once you get used to them) whether that's tracepoints or just
strategically placed noinline functions.

Thanks.

-- 
tejun