From: Cristian Marussi <firstname.lastname@example.org> To: Florian Fainelli <email@example.com> Cc: firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, Jonathan.Cameron@Huawei.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: Re: [PATCH 3/4] firmware: arm_scmi: Introduce monotonically increasing tokens Date: Wed, 26 May 2021 15:44:28 +0100 [thread overview] Message-ID: <20210526144428.GO28060@e120937-lin> (raw) In-Reply-To: <email@example.com> Hi Florian, On Mon, May 24, 2021 at 07:13:35PM -0700, Florian Fainelli wrote: > > > On 5/24/2021 4:15 PM, Cristian Marussi wrote: > > Tokens are sequence numbers embedded in the each SCMI message header: they > > are used to correlate commands with responses (and delayed responses), but > > their usage and policy of selection is entirely up to the caller (usually > > the OSPM agent), while they are completely opaque to the callee (SCMI > > server platform) which merely copies them back from the command into the > > response message header. > > This also means that the platform does not, can not and should not enforce > > any kind of policy on received messages depending on the contained sequence > > number: platform can perfectly handle concurrent requests carrying the same > > identifiying token if that should happen. > > > > Moreover the platform is not required to produce in-order responses to > > agent requests, the only constraint in these regards is that in case of > > an asynchronous message the delayed response must be sent after the > > immediate response for the synchronous part of the command transaction. > > > > Currenly the SCMI stack of the OSPM agent selects as token for the > > s/as token/a token/? > > > egressing commands the lowest possible number which is not already in use > > by an existing in-flight transaction, which means, in other words, that > > we immediately reuse any token after its transaction has completed or it > > has timed out: this indeed simplifies token and associated xfer management > > and lookup. > > > > Under the above assumptions and constraints, since there is really no state > > shared between the agent and the platform to let the platform know when a > > token and its associated message has timed out, the current policy of early > > reuse of tokens can easily lead to the situation in which a spurios or late > > s/spurios/spurious/ > > > received response (or delayed_response), related to an old stale and timed > > out transaction, can be wrongly associated to a newer valid in-flight xfer > > that just happens to have reused the same token. > > > > This misbehavior on such ghost responses is more easily exposed on those > > transports that naturally have an higher level of parallelism in processing > > multiple concurrent in-flight messages. > > > > This commit introduces a new policy of selection of tokens for the OSPM > > agent: each new transfer now gets the next available and monotonically > > increasing token, until tokens are exhausted and the counter rolls over. > > > > Such new policy mitigates the above issues with ghost responses since the > > tokens are now reused as later as possible (when they roll back ideally) > > and so it is much easier to identify ghost responses to stale timed out > > transactions: this also helps in simplifying the specific transports > > implementation since stale transport messages can be easily identified > > and discarded early on in the rx path without the need to cross check > > their actual sate with the core transport layer. > > This mitigation is even more effective when, as is usual the case, the > > s/usual/usually/ > > > maximum number of pending messages is capped by the platform to a much > > lower value than whole possible range of tokens.(2^10) > > > > This internal policy change in the core SCMI transport layer is fully > > transparent to the specific transports so it has not and should not have > > any impact on the transports implementation. > > > > The empirically observed cost of such new procedure of token selection > > amounts in the best case to ~10us out of an observed full transaction cost > > of 3ms for the completion of a synchronous sensor reading command on a > > platform supporting commmands completion interrupts. > > s/commmands/commands/ > > > > > Signed-off-by: Cristian Marussi <firstname.lastname@example.org> > > Overall this looks good to me and is more straightforward than I thought. > [snip] > > > +/** > > + * scmi_xfer_token_set - Reserve and set new token for the xfer at hand > > + * > > + * @minfo: Pointer to Tx/Rx Message management info based on channel type > > + * @xfer: The xfer to act upon > > + * > > + * Pick the next unused monotonically increasing token and set it into > > + * xfer->hdr.seq: picking a monotonically increasing value avoids reusing > > + * immediately tokens of just completed or timed-out xfers, mitigating the risk > > + * of wrongly associating a late received answer for an expired xfer to a live > > + * in-flight transaction which happened to have reused the same token. > > This was a bit harder to read than I thought, how about: > > picking a monotonically increasing value avoids immediate reuse of > freshly completed or timed-out xfers, thus mitigating the risk of > incorrect association of a late and expired xfer with a live in-flight > transaction, both happening to re-use the same token identifier. > -- > Florian Thanks for having a look and for the feedback ! I'll fix you remarks in V2. Thanks, Cristian _______________________________________________ linux-arm-kernel mailing list email@example.com http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-26 15:56 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-24 23:14 [PATCH 0/4] Review/Extend SCMI Transport Core layer Cristian Marussi 2021-05-24 23:15 ` [PATCH 1/4] firmware: arm_scmi: reset_rx_to_maxsz during async commands Cristian Marussi 2021-05-24 23:15 ` [PATCH 2/4] firmware: arm_scmi: Add support for type handling in common functions Cristian Marussi 2021-05-25 1:53 ` Florian Fainelli 2021-05-24 23:15 ` [PATCH 3/4] firmware: arm_scmi: Introduce monotonically increasing tokens Cristian Marussi 2021-05-25 2:13 ` Florian Fainelli 2021-05-26 14:44 ` Cristian Marussi [this message] [not found] ` <CA+-6iNx-EoMUhOncrgYTxh52mW_7yjjBbfHK8mVyEY0Uw4piwg@mail.gmail.com> 2021-05-26 14:45 ` Cristian Marussi 2021-05-24 23:15 ` [PATCH 4/4] firmware: arm_scmi: Introduce delegated xfers support Cristian Marussi 2021-05-25 2:20 ` Florian Fainelli 2021-05-26 14:53 ` Cristian Marussi
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