From: Florian Fainelli <firstname.lastname@example.org> To: Cristian Marussi <email@example.com>, firstname.lastname@example.org, email@example.com Cc: 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: Mon, 24 May 2021 19:13:35 -0700 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> 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 <email@example.com> 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 _______________________________________________ linux-arm-kernel mailing list firstname.lastname@example.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2021-05-25 3:41 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 [this message] 2021-05-26 14:44 ` Cristian Marussi [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
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --email@example.com \ --firstname.lastname@example.org \ --cc=Jonathan.Cameron@Huawei.com \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --email@example.com \ --firstname.lastname@example.org \ --subject='Re: [PATCH 3/4] firmware: arm_scmi: Introduce monotonically increasing tokens' \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: link
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for NNTP newsgroup(s).