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[104.199.75.203]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-42bb6da24bbsm233630715e9.0.2024.09.05.06.17.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Sep 2024 06:17:46 -0700 (PDT) Date: Thu, 5 Sep 2024 14:17:42 +0100 From: Vincent Donnefort To: John Stultz Cc: rostedt@goodmis.org, mhiramat@kernel.org, linux-trace-kernel@vger.kernel.org, maz@kernel.org, oliver.upton@linux.dev, kvmarm@lists.linux.dev, will@kernel.org, qperret@google.com, kernel-team@android.com, Thomas Gleixner Subject: Re: [RFC PATCH 04/11] timekeeping: Export the boot clock in snapshots Message-ID: References: <20240805173234.3542917-1-vdonnefort@google.com> <20240805173234.3542917-5-vdonnefort@google.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Aug 22, 2024 at 11:13:11AM -0700, John Stultz wrote: > On Mon, Aug 5, 2024 at 10:33 AM 'Vincent Donnefort' via kernel-team > wrote: > > > > On arm64 systems, the arch timer can be accessible by both EL1 and EL2. > > This means when running with nVHE or protected KVM, it is easy to > > generate clock values from the hypervisor, synchronized with the kernel. > > > > For tracing purpose, the boot clock is interesting as it doesn't stop on > > suspend. Export it as part of the time snapshot. This will later allow > > the hypervisor to add boot clock timestamps to its events. > > > > Signed-off-by: Vincent Donnefort > > > > diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h > > index fc12a9ba2c88..0fc6a61d64bd 100644 > > --- a/include/linux/timekeeping.h > > +++ b/include/linux/timekeeping.h > > @@ -275,18 +275,24 @@ struct ktime_timestamps { > > * counter value > > * @cycles: Clocksource counter value to produce the system times > > * @real: Realtime system time > > + * @boot: Boot time > > So, adding the boottime to this kernel-internal snapshot seems reasonable to me. > > > * @raw: Monotonic raw system time > > * @cs_id: Clocksource ID > > * @clock_was_set_seq: The sequence number of clock-was-set events > > * @cs_was_changed_seq: The sequence number of clocksource change events > > + * @mono_shift: The monotonic clock slope shift > > + * @mono_mult: The monotonic clock slope mult > > > This bit, including the mult/shift pair however, isn't well explained > and is a little more worrying. > > > > @@ -1074,14 +1076,21 @@ void ktime_get_snapshot(struct system_time_snapshot *systime_snapshot) > > systime_snapshot->clock_was_set_seq = tk->clock_was_set_seq; > > base_real = ktime_add(tk->tkr_mono.base, > > tk_core.timekeeper.offs_real); > > + base_boot = ktime_add(tk->tkr_mono.base, > > + tk_core.timekeeper.offs_boot); > > base_raw = tk->tkr_raw.base; > > nsec_real = timekeeping_cycles_to_ns(&tk->tkr_mono, now); > > nsec_raw = timekeeping_cycles_to_ns(&tk->tkr_raw, now); > > + mono_mult = tk->tkr_mono.mult; > > + mono_shift = tk->tkr_mono.shift; > > } while (read_seqcount_retry(&tk_core.seq, seq)); > > > > systime_snapshot->cycles = now; > > systime_snapshot->real = ktime_add_ns(base_real, nsec_real); > > + systime_snapshot->boot = ktime_add_ns(base_boot, nsec_real); > > systime_snapshot->raw = ktime_add_ns(base_raw, nsec_raw); > > + systime_snapshot->mono_shift = mono_shift; > > + systime_snapshot->mono_mult = mono_mult; > > } > > EXPORT_SYMBOL_GPL(ktime_get_snapshot); > > So this looks like you're trying to stuff kernel timekeeping internal > values into the snapshot so you can skirt around the timekeeping > subsystem and generate your own timestamps. > > This ends up duplicating logic, but in an incomplete way. For > instance, you don't have things like ntp state, etc, so the timestamps > you generate will not exactly match the kernel, and may have > discontinuities. :( > > Now for many cases "close enough" is fine. But the difficulty is the > expectation bar always raises, and eventually "close enough" isn't and > we have a broken interface that has to be fixed. > > That said, I do get the need to have something like this is > legitimate. There have been a number of cases where external hardware > (PTP timestamps from NICs) or contexts (virt) are able to record > hardware clocksource timestamps on their own, and want to be able to > map that back to the kernel's (or maybe "a kernel's" if there are > multiple VMs) sense of time. Sometimes even wanting to do this quite > a bit later after the timestamp was recorded. The ktime_get_snapshot() > logic was added in the first place for this reason. > > Some more aggressive approaches try to dump a bunch of the internal > kernel timekeeping state out to userland and call it an api. > See https://lore.kernel.org/lkml/410bbef9771ef8aa51704994a70d5965e367e2ce.camel@infradead.org/ > for a recent (and thorough) effort there. > > I'm very much not a fan of this approach, as it mimics older efforts > for userspace time calculations that were done before we settled on > VDSOs, which were very fragile and required years of keeping backwards > compatibility logic to map the current kernel state back to separate > structures and expensive conversions to different units that userland > expected. > > The benefit with VDSO interface is while the data is exposed to > userland, the structure is not, and the logic is still kernel > controlled, so changes to internal state can be done without breaking > userland. > > Something I have been thinking about is maybe it would be beneficial > to rework the timekeeping core so that given a clocksource timestamp, > it could calculate the time for that timestamp. While existing apis > would still do a new read of the clocksource, so the timestamps would > always increase, an old timestamp could be used to retro-calculate a > past time. The thing that prevents this now is that the timekeeping > core doesn't keep any history, so we can't correctly back-calculate > times before the last state change. But potentially we could keep a > buffer of timekeeper states associated with clocksource intervals, and > so we could find the right state to use for a given clocksource > timestamp. Now, this would still only work to a point, as we don't > want to keep tons of historical state. But then with this, maybe we > could switch to something more VDSO-like where the PTP drivers or host > systems could request a time given a timestamp (and probably some > clocksource id so we can sanity check everyone is using the same > clock), and we could still provide what they want without having to > expose all of our state. > > Unfortunately though, this is all hand waving and pontificating on my > part, as it would be a large rework. But it seems something closer > where we share opaque kernel state along with logic with proper > syscall like APIs to do the calculations, would be a much better > approach over just exporting more kernel state as an API. > > For a more short term approach, since you can't be exact outside of > the timekeeping logic, why not interpolate from the data > ktime_get_snapshot already provides to calculate your own sense of the > frequency? Understood, I shouldn't sneak out mult and shift. So for the following version, I'll just use the boot clock value and process my "own" mult and "shift". Thanks for having a look at the change! > > thanks > -john