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 4EDFD6F2E8; Thu, 12 Sep 2024 20:18:46 +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=1726172326; cv=none; b=SjqO/XagaqZ+goXHCKWyqyiGat0Lr+UTgMz/KBqmrdLnd+08EQil3FLstvlr35pyji/ou7Bg+X5pugepvX80pW3XGY+qI32yhP75UujqSuxyE8CGIKgFoso6VvQDxhv9/PteS1n4G3mRsgf80ub7qTNA46WpnFo2bjrzMb04ZT4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1726172326; c=relaxed/simple; bh=HTEWn8xAj3rJy2FUXtz5csfl5vK7avmJrmLXt1tp31Q=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=O5vMfsjt2dAdjNGTRjgIjwCwscLs7z/w90jHRLVMmSwygGxPXsAAFyiH0xFG+6ACufwDJ1cpm2d+3OnZOOQWwDONwfx6hpyXIDsME/Tklin4t0S5fF2JTAMiemyURJUXq/grQm8rwF1vCJ/leUpG/WlYG12qCzeihed49WnH4HQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=UwM6lTEw; 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="UwM6lTEw" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 28895C4CEC3; Thu, 12 Sep 2024 20:18:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1726172326; bh=HTEWn8xAj3rJy2FUXtz5csfl5vK7avmJrmLXt1tp31Q=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=UwM6lTEwqZKF1a2IX2ZFs9ZCnF7Ov5/klGP5quBhCcSc7EvKkCUBND1sCMSetq/Bn oCQ0UClnrilmNqBdz9LNHDkcU5TsRwr/uv0QRD34iUmYhqcmr69x2lNYQuoTrBIUOn TKLLN+hJG2EMGiCORNscbX6Xglk6XPl+PZZPE/hRxPGWNulAdFLJJjMgVWZ//TnzTv QYlt4TewIUe11ncZ8hEyUTxGIprIHK+9e/4RlmeVWG1s/itwj+zZ3y9c1sRP0reG0G BBehitHaZn0U6tqPDFEfKGwxa33tjiqS1RY5N+bKfRbjPmRVyKmDWH+nS6R1G4DEFd DcGiittwJv8kQ== Message-ID: <80f8f49da3f7208101e497a130a8abe106b298ad.camel@kernel.org> Subject: Re: [PATCH v2] timekeeping: move multigrain timestamp floor handling into timekeeper From: Jeff Layton To: John Stultz Cc: Alexander Viro , Christian Brauner , Jan Kara , Thomas Gleixner , Stephen Boyd , Arnd Bergmann , Vadim Fedorenko , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, kernel test robot Date: Thu, 12 Sep 2024 16:18:43 -0400 In-Reply-To: References: <20240912-mgtime-v2-1-54db84afb7a7@kernel.org> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv egyjnSsFt7EGoDjdKqr1TS9syJYFjagYtvWk/UfHlW09X+jOh4vYtfX7iYSx/NfqV3W1D7EDi0PqV 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1.0 On Thu, 2024-09-12 at 13:11 -0700, John Stultz wrote: > On Thu, Sep 12, 2024 at 11:02=E2=80=AFAM Jeff Layton = wrote: > >=20 > > The kernel test robot reported a performance hit in some will-it-scale > > tests due to the multigrain timestamp patches. My own testing showed > > about a 7% drop in performance on the pipe1_threads test, and the data > > showed that coarse_ctime() was slowing down current_time(). >=20 > So, you provided some useful detail about why coarse_ctime() was slow > in your reply earlier, but it would be good to preserve that in the > commit message here. >=20 >=20 > > Move the multigrain timestamp floor tracking word into timekeeper.c. Ad= d > > two new public interfaces: The first fills a timespec64 with the later > > of the coarse-grained clock and the floor time, and the second gets a > > fine-grained time and tries to swap it into the floor and fills a > > timespec64 with the result. > >=20 > > The first function returns an opaque cookie that is suitable for passin= g > > to the second, which will use it as the "old" value in the cmpxchg. >=20 > The cookie usage isn't totally clear to me right off. It feels a bit > more subtle then I'd expect. >=20 >=20 > > diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c > > index 5391e4167d60..bb039c9d525e 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; > > + >=20 > So I do really like this general approach of having an internal floor > value combined with special coarse/fine grained assessors that work > with the floor, so we're not impacting the normal hotpath logic > (basically I was writing up a suggestion to this effect to the thread > with Arnd when I realized you had follow up patch in my inbox). >=20 >=20 > > 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 +2401,76 @@ 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 to pass > > + * 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); >=20 > Generally this looks ok to me. >=20 >=20 > > +/** > > + * 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 > Again this cookie argument usage and the behavior of this function > isn't very clear to me. >=20 > > +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 { > > + *ts =3D ktime_to_timespec64(ktime_add(old, offset)); > > + } > > + > > +} > > +EXPORT_SYMBOL(ktime_get_real_ts64_mg); >=20 >=20 > So initially I was expecting this to look something like (sorry for > the whitespace damage here): > { > 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)); >=20 > mono =3D ktime_add_ns(mono, nsecs); > do { > old =3D atomic64_read(&mg_floor); > if (floor >=3D mono) > break; > } while(!atomic64_try_cmpxchg(&mg_floor, old, mono); > ts->tv_nsec =3D 0; > timespec64_add_ns(ts, nsecs); > } >=20 > Where you read the tk data, atomically update the floor (assuming it's > not in the future) and then return the finegrained value, not needing > to manage a cookie value. >=20 > But instead, it seems like if something has happened since the cookie > value was saved (another cpu getting a fine grained timestamp), your > ktime_get_real_ts64_mg() will fall back to returning the same coarse > grained time saved to the cookie, as if no time had past? > > It seems like that could cause problems: >=20 > cpu1 cpu2 > -------------------------------------------------------------------------= - > t2a =3D ktime_get_coarse_real_ts= 64_mg > t1a =3D ktime_get_coarse_real_ts64_mg() > t1b =3D ktime_get_real_ts64_mg(t1a) >=20 > t2b =3D ktime_get_real_ts64_mg(t= 2a) >=20 > Where t2b will seem to be before t1b, even though it happened afterwards. >=20 Ahh no, the subtle thing about atomic64_try_cmpxchg is that it overwrites "old" with the value that was currently there in the event that the cmp fails. So, the try_cmpxchg there will either swap the new value into place, or if it was updated in the meantime, "old" will now refer to the value that's currently in the floor word. Either is fine in this case, so we don't need to retry anything. --=20 Jeff Layton