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 5B7E4BA49; Sat, 14 Sep 2024 23:14:10 +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=1726355650; cv=none; b=JiAfVRTYCIYBOTCg5z+2V/iZqo6/cI/WMQBj1yz6oNieFE3QZ6KBhANvwRRo0qXHHRvKsxZ9YStqy5doOVq7u170ozKoCaiD5xhVl3xwY2EGBlf3ZLkofFRtOV5QAu0eF3f+w9rzseGfLgCK7f9hnjAhwdqDKaWtb/ZlDzdgatE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726355650; c=relaxed/simple; bh=Tlf3ImrTd3tKtsB4NFTexVCJZGdlq1aFiKphnW52w6o=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=LDwhsaNlz+K4Ne1W3Ee8oafrY7Om77NFC7teYpJyIVlFonJBoTx8ZtvUynPpLmYiLJdWLDLitHl9wRUaB6fq4qawOE5zAGNnLlg5hgj69F/dMykTYL4XhMOo1MthnI/0OcsrriZVgiq9dYPPMDK08tXjgW7/Q2ev6itN+tlGAyE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BsY7qSbS; 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="BsY7qSbS" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4FD42C4CEC0; Sat, 14 Sep 2024 23:14:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726355649; bh=Tlf3ImrTd3tKtsB4NFTexVCJZGdlq1aFiKphnW52w6o=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=BsY7qSbS/NUdA3Wwt8/6uhL91WzrSGHZq4tPn1LcKvCPB3S62LXop13Kg+qd9mHOm IM+Iyxx/D+2oAw4Nv1dgozyAEtyvPQZg/5qx9gGHedbdf6mKArsOpreAr6meZvIltH X0H+J4kxN5y9rNES1PIXA8hLWZOBmuKgL+CS3L0nRC/+ZCHfv/eSVLnqsDL1CezJzF yLNm8YWQZzb6Z9Qrxbv6ZfOxQZXMhtI75Bo4/CBOiHq/X6hB6/e9a6yz2gDyIGGljv knHzaTskLy7fwdpRgB/39ONiHLG6eRs3jnpdZmJ5+1M3aR9ZqOjGY0nY7a1Wxcs8Id mYLOsYJF5poig== Message-ID: Subject: Re: [PATCH v8 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 , Randy Dunlap , 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: Sat, 14 Sep 2024 19:14:06 -0400 In-Reply-To: References: <20240914-mgtime-v8-0-5bd872330bed@kernel.org> <20240914-mgtime-v8-1-5bd872330bed@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.fc40app2) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Sat, 2024-09-14 at 13:10 -0700, John Stultz wrote: > On Sat, Sep 14, 2024 at 10:07=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 > > 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 > This last bit seems out of date. >=20 Thanks. Dropped the last paragraph. > > --- > > include/linux/timekeeping.h | 4 +++ > > kernel/time/timekeeping.c | 82 +++++++++++++++++++++++++++++++++++++= ++++++++ > > 2 files changed, 86 insertions(+) > >=20 > > diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h > > index fc12a9ba2c88..7aa85246c183 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 void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts); > > +extern void ktime_get_real_ts64_mg(struct timespec64 *ts); > > + > > void getboottime64(struct timespec64 *ts); > >=20 > > /* > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > > index 5391e4167d60..16937242b904 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 +2401,81 @@ 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. Note that this is a filesystem-specific > > + * interface and should be avoided outside of that context. > > + */ > > +void 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); > > +} > > +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 > > + * > > + * Get a current monotonic fine-grained time value and attempt to swap > > + * it into the floor. @ts will be filled with the resulting floor valu= e, > > + * regardless of the outcome of the swap. Note that this is a filesyst= em > > + * specific interface and should be avoided outside of that context. > > + */ > > +void ktime_get_real_ts64_mg(struct timespec64 *ts, u64 cookie) >=20 > Still passing a cookie. It doesn't match the header definition, so I'm > surprised this builds. >=20 Yeah, I didn't see a warning when I built it. Luckily the extra parameter is ignored anyway, so it no harm. I've fixed that up in my tree. > > +{ > > + struct timekeeper *tk =3D &tk_core.timekeeper; > > + ktime_t old =3D atomic64_read(&mg_floor); > > + ktime_t offset, mono; > > + 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. "old" has now been updated with the > > + * current value of mg_floor, so use that to return > > + * the current coarse floor value. > > + */ > > + *ts =3D ktime_to_timespec64(ktime_add(old, offset)); > > + } > > +} > > +EXPORT_SYMBOL_GPL(ktime_get_real_ts64_mg); >=20 > Other than those issues, I'm ok with it. Thanks again for working > through my concerns! >=20 > Since I'm traveling for LPC soon, to save the next cycle, once the > fixes above are sorted: > Acked-by: John Stultz >=20 Thanks for the review! --=20 Jeff Layton