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 CED9E224CF; Fri, 13 Sep 2024 19:06:55 +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=1726254416; cv=none; b=GBZErquQ7CH9uRZ+y/xwXRrF8zrYN+21jHICH0fFKU79HTpEh+lx1fY5zRh2V6w65zLQx9w/e7/YfkaTgEbxCQAsxXeE6md57BDG3i9oI0Ew5AwCtDXPESXitvNMWJRiPmHYp44mF5EUxH11XtskhYcf1Jm5BJqwBlSsz5zv+Y8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726254416; c=relaxed/simple; bh=IVp8l6c6f41YARzik52VEPuV2gqwV+A4TSgezp3UTFY=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=oUIvsrPhl/ABFQ9Qp903VUPlJGjM0pi1P23BDrjNkQCfmnvbaaM39mb0TVYJFzROunzM6e83+zzpq32ooFCUv/gszC8vQrQ0WViZP6fZTFHEbsr8/BUuIYeyVou3gFgnraZXbzujPvBO9LLlxzWJrGdS/UXYpK6VWuPf2n1oRzk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=rxiRFoYY; 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="rxiRFoYY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ECFADC4CEC0; Fri, 13 Sep 2024 19:06:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726254415; bh=IVp8l6c6f41YARzik52VEPuV2gqwV+A4TSgezp3UTFY=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=rxiRFoYY4/kzc8OfApKxV6U7AY4Xpi99RSsWygXYv8i4olbPdx3/x/7kPoTEfUbzg YhSt9Kt+hOCSLCFqlS5GGMenDanxz7Ts+B0x48Yee1o+3OUNTQVcIBpZxCRJMr8ssm ccc8bmd0eefFzmEKYxr4U4n4s5XMrlNQmDJW31cxIk1TVT7zBDlbQI1ePuL4PTffp5 hewtBg86TqpwbDKs564Xj5AOpS0UHgNe6II7qFx1FJOfGHZtM0vVZ/ToPjJdWAQbX5 qkDYIDIA711JYHbdWGS/tmMYlNk/v+HnF5MQJdt0UHkIP3y3nr02KGo6EF+p8i4J5T gIn6i4NFtE0YQ== Message-ID: <8756a43fe0b7c3f418b351adb05e7a146d33bdfe.camel@kernel.org> Subject: Re: [PATCH v7 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper From: Jeff Layton To: John Stultz Cc: Thomas Gleixner , Stephen Boyd , Alexander Viro , Christian Brauner , Jan Kara , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Jonathan Corbet , Chandan Babu R , "Darrick J. Wong" , Theodore Ts'o , Andreas Dilger , Chris Mason , Josef Bacik , David Sterba , Hugh Dickins , Andrew Morton , Chuck Lever , Vadim Fedorenko , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-xfs@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org Date: Fri, 13 Sep 2024 15:06:51 -0400 In-Reply-To: References: <20240913-mgtime-v7-0-92d4020e3b00@kernel.org> <20240913-mgtime-v7-1-92d4020e3b00@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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2tAMJUvFTdDbfe3zKlPDTuVsyNsALBGclPLagJ5ZTP+Vp2irAN9uwBuacBOTtmOdz4ZN2tdvNgozz uxp4CHBDVzAslUi2idy+xpsp47DWPxYFIRP3M8QG/aNW052LaPc0cedYxp8+9eiVUNpxF4SiU4i9J DfX/sn9XcfoVZIxMpCRE750zvJvcCUz9HojsrMQ1NFc7MFT1z3MOW2/RlzPcog7xvR5ENPH19ojRD CHqumUHRry+RF0lH00clzX/W8OrQJZtoBPXv9ahka/Vp7kEulcBJr1cH5Wz/WprhsIM7U9pse1f1g Yy9YbXtWctUz8uvDR7shsQxAhX3qO7DilMtuGo1v97I/Kx4gXQ52syh/w6EBny71CZrOgD6kJwPVV AaM1LRC28muq91WCFhs/nzHozpbzcheyGtMUI2Ao4K6mnY+3zIuXPygZMFr9KXE6fF7HzKxKuZMJO aEZCiDOq0anx6FmOzs5E6Jqdpo/mtI8beK+BE7Va6ni7YrQlnT0i3vaTVMTiCThbqsB20VrbMjlhp f8lfK1XVNbRq/R7GZ9zHESlsa35ha60yd/j3pu5hT2xyy8krV8vGhHvnJ1XRMJBAB/UYb6FyC7S+m QZIQXVeAA+smfTT0tDrisj1U5x6ZB9b3nBg65kc= Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.52.4 (3.52.4-1.fc40) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Fri, 2024-09-13 at 11:59 -0700, John Stultz wrote: > On Fri, Sep 13, 2024 at 6:54=E2=80=AFAM Jeff Layton = wrote: > >=20 > > For multigrain timestamps, we must keep track of the latest timestamp > > that has ever been handed out, and never hand out a coarse time below > > that value. > >=20 > > Add a static singleton atomic64_t into timekeeper.c that we can use to > > keep track of the latest fine-grained time ever handed out. This is >=20 > Maybe drop "ever" and add "handed out through a specific interface", > as timestamps can be accessed in a lot of ways that don't keep track > of what was returned. >=20 Will do. I'll make it clear that this only applies to the *_mg interfaces. >=20 > > tracked as a monotonic ktime_t value to ensure that it isn't affected b= y > > clock jumps. > >=20 > > Add two new public interfaces: > >=20 > > - ktime_get_coarse_real_ts64_mg() fills a timespec64 with the later of = the > > coarse-grained clock and the floor time > >=20 > > - ktime_get_real_ts64_mg() gets the fine-grained clock value, and tries > > to swap it into the floor. A timespec64 is filled with the result. > >=20 > > Since the floor is global, we take great pains to avoid updating it > > unless it's absolutely necessary. If we do the cmpxchg and find that th= e > > value has been updated since we fetched it, then we discard the > > fine-grained time that was fetched in favor of the recent update. > >=20 > > To maximize the window of this occurring when multiple tasks are racing > > to update the floor, ktime_get_coarse_real_ts64_mg returns a cookie > > value that represents the state of the floor tracking word, and > > ktime_get_real_ts64_mg accepts a cookie value that it uses as the "old" > > value when calling cmpxchg(). > >=20 > > Signed-off-by: Jeff Layton > > --- > > include/linux/timekeeping.h | 4 +++ > > kernel/time/timekeeping.c | 81 +++++++++++++++++++++++++++++++++++++= ++++++++ > > 2 files changed, 85 insertions(+) > >=20 > > diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h > > index fc12a9ba2c88..cf2293158c65 100644 > > --- a/include/linux/timekeeping.h > > +++ b/include/linux/timekeeping.h > > @@ -45,6 +45,10 @@ extern void ktime_get_real_ts64(struct timespec64 *t= v); > > extern void ktime_get_coarse_ts64(struct timespec64 *ts); > > extern void ktime_get_coarse_real_ts64(struct timespec64 *ts); > >=20 > > +/* Multigrain timestamp interfaces */ > > +extern u64 ktime_get_coarse_real_ts64_mg(struct timespec64 *ts); > > +extern void ktime_get_real_ts64_mg(struct timespec64 *ts, u64 cookie); > > + > > void getboottime64(struct timespec64 *ts); > >=20 > > /* > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > > index 5391e4167d60..ee11006a224f 100644 > > --- a/kernel/time/timekeeping.c > > +++ b/kernel/time/timekeeping.c > > @@ -114,6 +114,13 @@ static struct tk_fast tk_fast_raw ____cacheline_a= ligned =3D { > > .base[1] =3D FAST_TK_INIT, > > }; > >=20 > > +/* > > + * This represents the latest fine-grained time that we have handed ou= t as a > > + * timestamp on the system. Tracked as a monotonic ktime_t, and conver= ted to the > > + * realtime clock on an as-needed basis. > > + */ > > +static __cacheline_aligned_in_smp atomic64_t mg_floor; > > + > > static inline void tk_normalize_xtime(struct timekeeper *tk) > > { > > while (tk->tkr_mono.xtime_nsec >=3D ((u64)NSEC_PER_SEC << tk->t= kr_mono.shift)) { > > @@ -2394,6 +2401,80 @@ void ktime_get_coarse_real_ts64(struct timespec6= 4 *ts) > > } > > EXPORT_SYMBOL(ktime_get_coarse_real_ts64); > >=20 > > +/** > > + * ktime_get_coarse_real_ts64_mg - get later of coarse grained time or= floor > > + * @ts: timespec64 to be filled > > + * > > + * Adjust floor to realtime and compare it to the coarse time. Fill > > + * @ts with the latest one. Returns opaque cookie suitable for passing > > + * to ktime_get_real_ts64_mg(). > > + */ > > +u64 ktime_get_coarse_real_ts64_mg(struct timespec64 *ts) > > +{ > > + struct timekeeper *tk =3D &tk_core.timekeeper; > > + u64 floor =3D atomic64_read(&mg_floor); > > + ktime_t f_real, offset, coarse; > > + unsigned int seq; > > + > > + WARN_ON(timekeeping_suspended); > > + > > + do { > > + seq =3D read_seqcount_begin(&tk_core.seq); > > + *ts =3D tk_xtime(tk); > > + offset =3D *offsets[TK_OFFS_REAL]; > > + } while (read_seqcount_retry(&tk_core.seq, seq)); > > + > > + coarse =3D timespec64_to_ktime(*ts); > > + f_real =3D ktime_add(floor, offset); > > + if (ktime_after(f_real, coarse)) > > + *ts =3D ktime_to_timespec64(f_real); > > + return floor; > > +} > > +EXPORT_SYMBOL_GPL(ktime_get_coarse_real_ts64_mg); > > + > > +/** > > + * ktime_get_real_ts64_mg - attempt to update floor value and return r= esult > > + * @ts: pointer to the timespec to be set > > + * @cookie: opaque cookie from earlier call to ktime_get_coarse_rea= l_ts64_mg() > > + * > > + * Get a current monotonic fine-grained time value and attempt to swap > > + * it into the floor using @cookie as the "old" value. @ts will be > > + * filled with the resulting floor value, regardless of the outcome of > > + * the swap. >=20 > I'd add more detail here to clarify that this can return a coarse > floor value if the cookie is stale. >=20 Sure, or I can just drop the cookie, if that's better. > > +void ktime_get_real_ts64_mg(struct timespec64 *ts, u64 cookie) > > +{ > > + struct timekeeper *tk =3D &tk_core.timekeeper; > > + ktime_t offset, mono, old =3D (ktime_t)cookie; > > + unsigned int seq; > > + u64 nsecs; > > + > > + WARN_ON(timekeeping_suspended); > > + > > + do { > > + seq =3D read_seqcount_begin(&tk_core.seq); > > + > > + ts->tv_sec =3D tk->xtime_sec; > > + mono =3D tk->tkr_mono.base; > > + nsecs =3D timekeeping_get_ns(&tk->tkr_mono); > > + offset =3D *offsets[TK_OFFS_REAL]; > > + } while (read_seqcount_retry(&tk_core.seq, seq)); > > + > > + mono =3D ktime_add_ns(mono, nsecs); > > + > > + if (atomic64_try_cmpxchg(&mg_floor, &old, mono)) { > > + ts->tv_nsec =3D 0; > > + timespec64_add_ns(ts, nsecs); > > + } else { > > + /* > > + * Something has changed mg_floor since "old" was > > + * fetched. That value is just as valid, so accept it. > > + */ >=20 > Mostly because I embarrassingly tripped over this in front of > everyone, I might suggest: > /* > * mg_floor was updated since the cookie was fetched, so the > * the try_cmpxchg failed. However try_cmpxchg updated old > * with the current mg_floor, so use that to return the current > * coarse floor value > */ >=20 > :) Will do. I've already added some comments to that effect that should help clarify things. > Additionally, for these two new interfaces, since they are so > specifically tuned to this particular need in the vfs, it might be > good to add a comments in the kerneldoc here that they are special > case interfaces for the vfs and should be avoided outside that space. >=20 > That probably would alleviate my main worries, and we can polish the > details around cookie or no cookie later if needed. >=20 Will do. Thanks for the review! --=20 Jeff Layton