From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BB7D54A2C; Fri, 13 Sep 2024 19:01:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726254080; cv=none; b=Ckn+fFSd4DnYzR7aTS9a6OuQapmoAEXF8qTk9xFoMIYDYsoSwBcteXA2GBg2WM5LKsT/dehTmHFRAe3c2f56UrWObIC30QCVd0JVqsQwADeCfJEfvFAzMkhDj+91MSsEsAD3hPKkBRyiN/LBW5nScPqbK31XcfEY7mks3npQgr4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726254080; c=relaxed/simple; bh=2i1ZG4ljUzAmz9IKHnQ6+AZ/x+IQqBP/mEBwZRNLuMo=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=KNZGmtx134u8dhqUBt792UVQ4TMJBJUf41Uqrx4JfBOj2n+eQyUs5TyF5uRwK9oKZY8o1j1VXfF1yBZr9G7FwZQ3XoKAPNhf/ur2wjZUBl390okrNwOEJk4RYy20lqAKa7bR/FJZMVOwm4/DPA4vCX5bfpNFLU00gtbFtablvFw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JjJ7Trer; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JjJ7Trer" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4EC51C4CEC7; Fri, 13 Sep 2024 19:01:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726254080; bh=2i1ZG4ljUzAmz9IKHnQ6+AZ/x+IQqBP/mEBwZRNLuMo=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=JjJ7TrerA16lQxN0X1DF3z7Qk4a5O8Yp7ifogVlTVLG/g1c+80czzcM+iW1k4RXyj JTii8WPw1xcKmPXO5BO7yvB89j2PWsSwUsHG7DkvYMkYa/6mLHpHdABiNicGxCIEe9 loPLsR1ELkQrUUlT8abZiBOD8MHJVuhBD9IFum97gb9RDX64xj3SwiJhPrL3A6ux32 9GMF6eg2TiIWBOtZ3BGTXbb2mMlcLHNUwB2CScuu1sQFI/ng4n8FRpqLS4rulkWpE7 jIKOM29ujaA/xj49W9H/+IOoSf8tcnkUE6WS4iTt1j9y/fCiuiuZL5+zaDK8r1Mlaq v7QkuIrG+SxIQ== Message-ID: <4e4b1096b322f2998b5e59a861c5fdad410c108a.camel@kernel.org> Subject: Re: [PATCH v2] timekeeping: move multigrain timestamp floor handling into timekeeper From: Jeff Layton To: John Stultz Cc: Jan Kara , Alexander Viro , Christian Brauner , Thomas Gleixner , Stephen Boyd , Arnd Bergmann , Vadim Fedorenko , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Date: Fri, 13 Sep 2024 15:01:18 -0400 In-Reply-To: References: <20240912-mgtime-v2-1-54db84afb7a7@kernel.org> <20240913112602.xrfdn7hinz32bhso@quack3> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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Fri, 2024-09-13 at 11:43 -0700, John Stultz wrote: > On Fri, Sep 13, 2024 at 5:01=E2=80=AFAM Jeff Layton = wrote: > > On Fri, 2024-09-13 at 13:26 +0200, Jan Kara wrote: > > > So what would be the difference if we did instead: > > >=20 > > > old =3D atomic64_read(&mg_floor); > > >=20 > > > and not bother with the cookie? AFAIU this could result in somewhat m= ore > > > updates to mg_floor (the contention on the mg_floor cacheline would b= e the > > > same but there would be more invalidates of the cacheline). OTOH thes= e > > > updates can happen only if max(current_coarse_time, mg_floor) =3D=3D > > > inode->i_ctime which is presumably rare? What is your concern that I'= m > > > missing? > > >=20 > >=20 > > My main concern is the "somewhat more updates to mg_floor". mg_floor is > > a global variable, so one of my main goals is to minimize the updates > > to it. There is no correctness issue in doing what you're saying above > > (AFAICT anyway), but the window of time between when we fetch the > > current floor and try to do the swap will be smaller, and we'll end up > > doing more swaps as a result. >=20 > Would it be worth quantifying that cost? >=20 There's a patch in the larger set that adds some percpu counters to count events. One of them was successful floor value swaps. I dropped that particular counter from the v7 set, but we could resurrect it. > > Do you have any objection to adding the cookie to this API? >=20 > My main concern is it is just a bit subtle. I found it hard to grok > (though I can be pretty dim sometimes, so maybe that doesn't count for > much :) > It seems if it were misused, the fine-grained accessor could > constantly return coarse grained results when called repeatedly with a > very stale cookie. >=20 > Further, the point about avoiding "too many" mg_floor writes is a > little fuzzy. It feels almost like folks would need to use the cookie > update as a tuning knob to balance the granularity of their timestamps > against the cost of the global mg_floor writes. So this probably needs > some clear comments to make it more obvious. >=20 Fair points. I don't have any hard numbers around it. I'm mainly just trying to do what I can to keep the floor swaps to an absolute minimum. This is a global value after all so we really are better off avoiding cache invalidations. That said, passing the cookie like this would only open the window a small amount. I can certainly drop that part of the interface. In the big scheme of things I doubt it'll make much difference in performance, and if it does we can always bring it back. If that sounds OK, I'll send a v8 (after some testing). I have some comment updates I'd like to add as well. --=20 Jeff Layton