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 49C32149C69; Wed, 11 Sep 2024 20:43:04 +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=1726087385; cv=none; b=XwRO7u2d1wsdT6rz9U89WN5kNWAKpyX3waixGGdyyGD9YzPUbu9NHsK5tSQTiiquYoiqPqK8+iRmrWxtPpCpQXLEjn3PKQ2g9hk9B7mpfQMtyZaIFoilJwQ/ymINhX58kAo8q5bKlNHi8J4EywqNFtVVcHgkT8lWeaHEitKHYSU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726087385; c=relaxed/simple; bh=5hLn/XvbruBIpLM88BBJs1JwaGcVKyKFVbu5lPnzDbA=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=o1QrnE//qqZ5hsFug1HlPKeL5Pr+sLjU7/JBJtw8qZ6JdeZXYQIX3TyiX3mVfkWkHwDLIzeZpTxmhvmCT86lCLdjMLMthSNQtrMxFKPXpWvopgAYE6+GGuObrQ39jFoYTQ8UhD0awODJszaI7P1TAB/nzpL3zwQgO3L6yqLyRkM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZUypFd7K; 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="ZUypFd7K" Received: by smtp.kernel.org (Postfix) with ESMTPSA id C747CC4CEC0; Wed, 11 Sep 2024 20:43:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726087384; bh=5hLn/XvbruBIpLM88BBJs1JwaGcVKyKFVbu5lPnzDbA=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=ZUypFd7KoJTt6ciRxtUC9jrczalpgmfkywg78hX/uw70HMOdYRJvi1MrvNaNFtoyQ SwP1pmC7h0qTG9cflCbihHu3KvE+afk/uMM1uyjwFNn6513WRfY0o+Sw4ygq8697SQ 2CHTRxh/auTatUA1IeFyQHXybPygOaOo3s5Z4gSE2UTCNUSkoV6GeoDaKxQQ07Fe7u jlq2yEBZD2K5tWKaiklQY10w6caviWlC2LORLpLHGoQ86TwkIvuoEprTl47FaFrSGw 3qrH3nowlU0LmQzbd86Iql/dZPt5xnHhEDiaE9rbC7HjvQZ4aIWnaXiYd6l9HSkbE3 0pYViLBKNVEKA== Message-ID: Subject: Re: [PATCH] timekeeping: move multigrain ctime floor handling into timekeeper From: Jeff Layton To: Arnd Bergmann , John Stultz Cc: Alexander Viro , Christian Brauner , Jan Kara , Thomas Gleixner , Stephen Boyd , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Date: Wed, 11 Sep 2024 16:43:02 -0400 In-Reply-To: References: <20240911-mgtime-v1-1-e4aedf1d0d15@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV T2h6v8i8YqsATFPwO4nuiTmL6I40ZofxVd+9wdRI4Db8yUNA4ZSP2nqLcLtFjClYRBoJvRWvsv4lm 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2024, at 19:55, John Stultz wrote: > > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > > > index 5391e4167d60..56b979471c6a 100644 >=20 > > My confusion comes from the fact it seems like that would mean you > > have to do all your timestamping with CLOCK_MONOTONIC (so you have a > > useful floor value that you're keeping), so I'm not sure I understand > > the utility of returning CLOCK_REALTIME values. I guess I don't quite > > see the logic where the floor value is updated here, so I'm guessing. >=20 > I think we could take this further and store the floor value > in the timekeeper itself rather than in a global variable > next to the caller. >=20 > And instead of storing the absolute floor value, it would > be enough to store the delta since the previous > update_wall_time(), which in turn can get updated by a > variant of ktime_get_real_ts64() and reset to zero during > update_wall_time(). > > timespec64_add_ns() over the traditional version, and the > fine-grained version needs to atomically update that value. > If the delta value has to be a 64-bit integer, there also > needs to be some serialization of the reader side, but I > think that can be done with read_seqcount_begin() . >=20 I think we'd have to track this delta as an atomic value and cmpxchg new values into place. The zeroing seems quite tricky to make race- free. Currently, we fetch the floor value early in the process and if it changes before we can swap a new one into place, we just take whatever the new value is (since it's just as good). Since these are monotonic values, any new value is still newer than the original one, so its fine. I'm not sure that still works if we're dealing with a delta that is siding upward and downward. Maybe it does though. I'll take a stab at this tomorrow and see how it looks. Thanks for the suggestion! --=20 Jeff Layton