Re: [PATCH v4 04/16] firmware: arm_scmi: Introduce monotonically increasing tokens

Peter Hilber
2021-07-01T08:42:53Z

I find `monotonically increasing tokens' misleading, since the token may
wrap around quite often during normal usage. How about `modulo
incrementing'?

On 11.06.21 18:59, 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 a token for the egressing
> commands picking 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 policy indeed does simplify management and lookup of tokens
> and associated xfers.
>
> 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 spurious or
> late 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 misbehaviour 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 late as possible (when they roll back ideally)
> and so it is much easier to identify such 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 state with the core transport layer.
> This mitigation is even more effective when, as is usually the case, the
> maximum number of pending messages is capped by the platform to a much
> lower number than the whole possible range of tokens values (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 commands completion interrupts.
>
> Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
> ---
> drivers/firmware/arm_scmi/common.h | 23 +++
> drivers/firmware/arm_scmi/driver.c | 243 +++++++++++++++++++++++++----
> 2 files changed, 233 insertions(+), 33 deletions(-)
>
> diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
> index 6bb734e0e3ac..e64c5ca9ee7c 100644
> --- a/drivers/firmware/arm_scmi/common.h
> +++ b/drivers/firmware/arm_scmi/common.h
> @@ -14,7 +14,10 @@
> #include <linux/device.h>
> #include <linux/errno.h>
> #include <linux/kernel.h>
> +#include <linux/hashtable.h>
> +#include <linux/list.h>
> #include <linux/module.h>
> +#include <linux/refcount.h>
> #include <linux/scmi_protocol.h>
> #include <linux/types.h>
>
> @@ -127,6 +130,21 @@ struct scmi_msg {
> size_t len;
> };
>
> +/**
> + * An helper macro to lookup an xfer from the @pending_xfers hashtable
> + * using the message sequence number token as a key.
> + */
> +#define XFER_FIND(__ht, __k) \
> +({ \
> + typeof(__k) k_ = __k; \
> + struct scmi_xfer *xfer_ = NULL; \
> + \
> + hash_for_each_possible((__ht), xfer_, node, k_) \
> + if (xfer_->hdr.seq == k_) \
> + break; \
> + xfer_; \

There is an extra space before the return value.

> +})
> +
> /**
> * struct scmi_xfer - Structure representing a message flow
> *
> @@ -138,6 +156,9 @@ struct scmi_msg {
> * buffer for the rx path as we use for the tx path.
> * @done: command message transmit completion event
> * @async_done: pointer to delayed response message received event completion
> + * @users: A refcount to track the active users for this xfer
> + * @node: An hlist_node reference used to store this xfer, alternatively, on
> + * the free list @free_xfers or in the @pending_xfers hashtable
> */
> struct scmi_xfer {
> int transfer_id;
> @@ -146,6 +167,8 @@ struct scmi_xfer {
> struct scmi_msg rx;
> struct completion done;
> struct completion *async_done;
> + refcount_t users;
> + struct hlist_node node;
> };
>
> struct scmi_xfer_ops;
> diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
> index 20f8f0581f3a..f0b20ddb24f4 100644
> --- a/drivers/firmware/arm_scmi/driver.c
> +++ b/drivers/firmware/arm_scmi/driver.c
> @@ -21,6 +21,7 @@
> #include <linux/io.h>
> #include <linux/kernel.h>
> #include <linux/ktime.h>
> +#include <linux/hashtable.h>
> #include <linux/list.h>
> #include <linux/module.h>
> #include <linux/of_address.h>
> @@ -65,19 +66,29 @@ struct scmi_requested_dev {
> struct list_head node;
> };
>
> +#define SCMI_PENDING_XFERS_HT_ORDER_SZ 9

Micro-optimization note: This increases struct scmi_info size to > 8 kiB
(on 64-bit architectures). A lower size of the hash table would be
enough for some transports.

> +
> /**
> * struct scmi_xfers_info - Structure to manage transfer information
> *
> - * @xfer_block: Preallocated Message array
> * @xfer_alloc_table: Bitmap table for allocated messages.
> * Index of this bitmap table is also used for message
> * sequence identifier.
> * @xfer_lock: Protection for message allocation
> + * @last_token: A counter to use as base to generate for monotonically
> + * increasing tokens.
> + * @free_xfers: A free list for available to use xfers. It is initialized with
> + * a number of xfers equal to the maximum allowed in-flight
> + * messages.
> + * @pending_xfers: An hashtable, indexed by msg_hdr.seq, used to keep all the
> + * currently in-flight messages.
> */
> struct scmi_xfers_info {
> - struct scmi_xfer *xfer_block;
> unsigned long *xfer_alloc_table;
> spinlock_t xfer_lock;
> + atomic_t last_token;
> + struct hlist_head free_xfers;
> + DECLARE_HASHTABLE(pending_xfers, SCMI_PENDING_XFERS_HT_ORDER_SZ);
> };
>
> /**
> @@ -203,6 +214,117 @@ void *scmi_notification_instance_data_get(const struct scmi_handle *handle)
> return info->notify_priv;
> }
>
> +/**
> + * 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 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.
> + *
> + * Since platform is NOT required to answer our request in-order we should
> + * account for a few rare but possible scenarios:
> + *
> + * - exactly 'next_token' may be NOT available so pick xfer_id >= next_token
> + * using find_next_zero_bit() starting from candidate next_token bit
> + *
> + * - all tokens ahead upto (MSG_TOKEN_ID_MASK - 1) are used in-flight but we
> + * are plenty of free tokens at start, so try a second pass using
> + * find_next_zero_bit() and starting from 0.
> + *
> + * X = used in-flight
> + *
> + * Normal
> + * ------
> + *
> + * |- xfer_id picked
> + * -----------+----------------------------------------------------------
> + * | | |X|X|X| | | | | | ... ... ... ... ... ... ... ... ... ... ...|X|X|
> + * ----------------------------------------------------------------------
> + * ^
> + * |- next_token
> + *
> + * Out-of-order pending at start
> + * -----------------------------
> + *
> + * |- xfer_id picked, last_token fixed
> + * -----+----------------------------------------------------------------
> + * |X|X| | | | |X|X| ... ... ... ... ... ... ... ... ... ... ... ...|X| |
> + * ----------------------------------------------------------------------
> + * ^
> + * |- next_token
> + *
> + *
> + * Out-of-order pending at end
> + * ---------------------------
> + *
> + * |- xfer_id picked, last_token fixed
> + * -----+----------------------------------------------------------------
> + * |X|X| | | | |X|X| ... ... ... ... ... ... ... ... ... ... |X|X|X||X|X|
> + * ----------------------------------------------------------------------
> + * ^
> + * |- next_token
> + *
> + * Context: Assumes to be called with @xfer_lock already acquired.
> + *
> + * Return: 0 on Success or error
> + */
> +static int scmi_xfer_token_set(struct scmi_xfers_info *minfo,
> + struct scmi_xfer *xfer)
> +{
> + unsigned long xfer_id, next_token;
> +
> + /* Pick a candidate monotonic token in range [0, MSG_TOKEN_MAX - 1] */
> + next_token = (atomic_inc_return(&minfo->last_token) &
> + (MSG_TOKEN_MAX - 1));
> +
> + /* Pick the next available xfer_id >= next_token */
> + xfer_id = find_next_zero_bit(minfo->xfer_alloc_table,
> + MSG_TOKEN_MAX, next_token);
> + if (xfer_id == MSG_TOKEN_MAX) {
> + /*
> + * After heavily out-of-order responses, there are no free
> + * tokens ahead, but only at start of xfer_alloc_table so
> + * try again from the beginning.
> + */
> + xfer_id = find_next_zero_bit(minfo->xfer_alloc_table,
> + MSG_TOKEN_MAX, 0);
> + /*
> + * Something is wrong if we got here since there can be a
> + * maximum number of (MSG_TOKEN_MAX - 1) in-flight messages
> + * but we have not found any free token [0, MSG_TOKEN_MAX - 1].
> + */
> + if (WARN_ON_ONCE(xfer_id == MSG_TOKEN_MAX))
> + return -ENOMEM;
> + }
> +
> + /* Update +/- last_token accordingly if we skipped some hole */
> + if (xfer_id != next_token)
> + atomic_add((int)(xfer_id - next_token), &minfo->last_token);
> +
> + /* Set in-flight */
> + set_bit(xfer_id, minfo->xfer_alloc_table);
> + xfer->hdr.seq = (u16)xfer_id;
> +
> + return 0;
> +}
> +
> +/**
> + * scmi_xfer_token_clear - Release the token
> + *
> + * @minfo: Pointer to Tx/Rx Message management info based on channel type
> + * @xfer: The xfer to act upon
> + */
> +static inline void scmi_xfer_token_clear(struct scmi_xfers_info *minfo,
> + struct scmi_xfer *xfer)
> +{
> + clear_bit(xfer->hdr.seq, minfo->xfer_alloc_table);
> +}
> +
> /**
> * scmi_xfer_get() - Allocate one message
> *
> @@ -212,36 +334,49 @@ void *scmi_notification_instance_data_get(const struct scmi_handle *handle)
> * Helper function which is used by various message functions that are
> * exposed to clients of this driver for allocating a message traffic event.
> *
> - * This function can sleep depending on pending requests already in the system
> - * for the SCMI entity. Further, this also holds a spinlock to maintain
> - * integrity of internal data structures.
> + * Picks an xfer from the free list @free_xfers (if any available), sets a
> + * monotonically increasing token and stores the inflight xfer into the
> + * @pending_xfers hashtable for later retrieval.
> + *
> + * The successfully initialized xfer is refcounted.
> + *
> + * Context: Holds @xfer_lock while manipulating @xfer_alloc_table and
> + * @free_xfers.
> *
> * Return: 0 if all went fine, else corresponding error.
> */
> static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
> struct scmi_xfers_info *minfo)
> {
> - u16 xfer_id;
> + int ret;
> + unsigned long flags;
> struct scmi_xfer *xfer;
> - unsigned long flags, bit_pos;
> - struct scmi_info *info = handle_to_scmi_info(handle);
>
> - /* Keep the locked section as small as possible */
> spin_lock_irqsave(&minfo->xfer_lock, flags);
> - bit_pos = find_first_zero_bit(minfo->xfer_alloc_table,
> - info->desc->max_msg);
> - if (bit_pos == info->desc->max_msg) {
> + if (hlist_empty(&minfo->free_xfers)) {
> spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> return ERR_PTR(-ENOMEM);
> }
> - set_bit(bit_pos, minfo->xfer_alloc_table);
> - spin_unlock_irqrestore(&minfo->xfer_lock, flags);
>
> - xfer_id = bit_pos;
> + /* grab an xfer from the free_list */
> + xfer = hlist_entry(minfo->free_xfers.first, struct scmi_xfer, node);
> + hlist_del_init(&xfer->node);
>
> - xfer = &minfo->xfer_block[xfer_id];
> - xfer->hdr.seq = xfer_id;
> - xfer->transfer_id = atomic_inc_return(&transfer_last_id);
> + /* Pick and set monotonic token */
> + ret = scmi_xfer_token_set(minfo, xfer);
> + if (!ret) {
> + hash_add(minfo->pending_xfers, &xfer->node, xfer->hdr.seq);
> + } else {
> + dev_err(handle->dev, "Failed to get monotonic token %d\n", ret);
> + hlist_add_head(&xfer->node, &minfo->free_xfers);
> + xfer = ERR_PTR(ret);
> + }
> + spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> +
> + if (!IS_ERR(xfer)) {
> + refcount_set(&xfer->users, 1);

Maybe it would be better to do this inside the lock, so that there is no
(unlikely) race with refcount_inc() in scmi_xfer_acquire().

> + xfer->transfer_id = atomic_inc_return(&transfer_last_id);
> + }
>
> return xfer;
> }
> @@ -252,6 +387,9 @@ static struct scmi_xfer *scmi_xfer_get(const struct scmi_handle *handle,
> * @minfo: Pointer to Tx/Rx Message management info based on channel type
> * @xfer: message that was reserved by scmi_xfer_get
> *
> + * After refcount check, possibly release an xfer, clearing the token slot,
> + * removing xfer from @pending_xfers and putting it back into free_xfers.
> + *
> * This holds a spinlock to maintain integrity of internal data structures.
> */
> static void
> @@ -259,16 +397,41 @@ __scmi_xfer_put(struct scmi_xfers_info *minfo, struct scmi_xfer *xfer)
> {
> unsigned long flags;
>
> - /*
> - * Keep the locked section as small as possible
> - * NOTE: we might escape with smp_mb and no lock here..
> - * but just be conservative and symmetric.
> - */
> spin_lock_irqsave(&minfo->xfer_lock, flags);
> - clear_bit(xfer->hdr.seq, minfo->xfer_alloc_table);
> + if (refcount_dec_and_test(&xfer->users)) {
> + scmi_xfer_token_clear(minfo, xfer);
> + hash_del(&xfer->node);
> + hlist_add_head(&xfer->node, &minfo->free_xfers);
> + }
> spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> }
>
> +/**
> + * scmi_xfer_lookup_unlocked - Helper to lookup an xfer_id
> + *
> + * @minfo: Pointer to Tx/Rx Message management info based on channel type
> + * @xfer_id: Token ID to lookup in @pending_xfers
> + *
> + * Refcounting is untouched.
> + *
> + * Context: Assumes to be called with @xfer_lock already acquired.
> + *
> + * Return: A valid xfer on Success or error otherwise
> + */
> +static struct scmi_xfer *
> +scmi_xfer_lookup_unlocked(struct scmi_xfers_info *minfo, u16 xfer_id)
> +{
> + struct scmi_xfer *xfer = NULL;
> +
> + if (xfer_id >= MSG_TOKEN_MAX)
> + return ERR_PTR(-EINVAL);
> +
> + if (test_bit(xfer_id, minfo->xfer_alloc_table))
> + xfer = XFER_FIND(minfo->pending_xfers, xfer_id);
> +
> + return xfer ?: ERR_PTR(-EINVAL);
> +}
> +
> static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
> {
> struct scmi_xfer *xfer;
> @@ -305,19 +468,22 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo, u32 msg_hdr)
> static void scmi_handle_response(struct scmi_chan_info *cinfo,
> u16 xfer_id, u8 msg_type)
> {
> + unsigned long flags;
> struct scmi_xfer *xfer;
> struct device *dev = cinfo->dev;
> struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
> struct scmi_xfers_info *minfo = &info->tx_minfo;
>
> /* Are we even expecting this? */
> - if (!test_bit(xfer_id, minfo->xfer_alloc_table)) {
> + spin_lock_irqsave(&minfo->xfer_lock, flags);
> + xfer = scmi_xfer_lookup_unlocked(minfo, xfer_id);
> + spin_unlock_irqrestore(&minfo->xfer_lock, flags);
> + if (IS_ERR(xfer)) {
> dev_err(dev, "message for %d is not expected!\n", xfer_id);
> info->desc->ops->clear_channel(cinfo);
> return;
> }
>
> - xfer = &minfo->xfer_block[xfer_id];
> /*
> * Even if a response was indeed expected on this slot at this point,
> * a buggy platform could wrongly reply feeding us an unexpected
> @@ -1033,18 +1199,25 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
> return -EINVAL;
> }
>
> - info->xfer_block = devm_kcalloc(dev, desc->max_msg,
> - sizeof(*info->xfer_block), GFP_KERNEL);
> - if (!info->xfer_block)
> - return -ENOMEM;
> + hash_init(info->pending_xfers);
>
> - info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(desc->max_msg),
> + /* Allocate a bitmask sized to hold MSG_TOKEN_MAX tokens */
> + info->xfer_alloc_table = devm_kcalloc(dev, BITS_TO_LONGS(MSG_TOKEN_MAX),
> sizeof(long), GFP_KERNEL);
> if (!info->xfer_alloc_table)
> return -ENOMEM;
>
> - /* Pre-initialize the buffer pointer to pre-allocated buffers */
> - for (i = 0, xfer = info->xfer_block; i < desc->max_msg; i++, xfer++) {
> + /*
> + * Preallocate a number of xfers equal to max inflight messages,
> + * pre-initialize the buffer pointer to pre-allocated buffers and
> + * attach all of them to the free list
> + */
> + INIT_HLIST_HEAD(&info->free_xfers);
> + for (i = 0; i < desc->max_msg; i++) {
> + xfer = devm_kzalloc(dev, sizeof(*xfer), GFP_KERNEL);
> + if (!xfer)
> + return -ENOMEM;
> +
> xfer->rx.buf = devm_kcalloc(dev, sizeof(u8), desc->max_msg_size,
> GFP_KERNEL);
> if (!xfer->rx.buf)
> @@ -1052,8 +1225,12 @@ static int __scmi_xfer_info_init(struct scmi_info *sinfo,
>
> xfer->tx.buf = xfer->rx.buf;
> init_completion(&xfer->done);
> +
> + /* Add initialized xfer to the free list */
> + hlist_add_head(&xfer->node, &info->free_xfers);
> }
>
> + atomic_set(&info->last_token, -1);
> spin_lock_init(&info->xfer_lock);
>
> return 0;
>