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NTB: ntb_transport: Add 'tx_memcpy_offload' module option

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Some platforms (e.g. R-Car S4) do not gain from using a DMAC on TX path
in ntb_transport and end up CPU-bound on memcpy_toio(). Add a module
parameter 'tx_memcpy_offload' that moves the TX memcpy_toio() and
descriptor writes to a per-QP kernel thread. It is disabled by default.

This change also fixes a rare ordering hazard in ntb_tx_copy_callback(),
that was observed on R-Car S4 once throughput improved with the new
module parameter: the DONE flag write to the peer MW, which is WC
mapped, could be observed after the DB/MSI trigger. Both operations are
posted PCIe MWr (often via different OB iATUs), so WC buffering and
bridges may reorder visibility. Insert dma_mb() to enforce store->load
ordering and then read back hdr->flags to flush the posted write before
ringing the doorbell / issuing MSI.

While at it, update tx_index with WRITE_ONCE() at the earlier possible
location to make ntb_transport_tx_free_entry() robust.

Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Jon Mason <jdmason@kudzu.us>
This commit is contained in:
Koichiro Den
2025-10-23 16:21:05 +09:00
committed by Jon Mason
parent b36490b5fb
commit 322617a06c
1 changed files with 100 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

104
drivers/ntb/ntb_transport.c
View File

@@ -54,12 +54,14 @@
#include <linux/errno.h>
#include <linux/export.h>
#include <linux/interrupt.h>
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/pci.h>
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/uaccess.h>
#include <linux/mutex.h>
#include <linux/wait.h>
#include "linux/ntb.h"
#include "linux/ntb_transport.h"
@@ -100,6 +102,10 @@ module_param(use_msi, bool, 0644);
MODULE_PARM_DESC(use_msi, "Use MSI interrupts instead of doorbells");
#endif
static bool tx_memcpy_offload;
module_param(tx_memcpy_offload, bool, 0644);
MODULE_PARM_DESC(tx_memcpy_offload, "Offload TX memcpy_toio() to a kernel thread");
static struct dentry *nt_debugfs_dir;
/* Only two-ports NTB devices are supported */
@@ -148,7 +154,9 @@ struct ntb_transport_qp {
void (*tx_handler)(struct ntb_transport_qp *qp, void *qp_data,
void *data, int len);
struct list_head tx_free_q;
struct list_head tx_offl_q;
spinlock_t ntb_tx_free_q_lock;
spinlock_t ntb_tx_offl_q_lock;
void __iomem *tx_mw;
phys_addr_t tx_mw_phys;
size_t tx_mw_size;
@@ -199,6 +207,9 @@ struct ntb_transport_qp {
int msi_irq;
struct ntb_msi_desc msi_desc;
struct ntb_msi_desc peer_msi_desc;
struct task_struct *tx_offload_thread;
wait_queue_head_t tx_offload_wq;
};
struct ntb_transport_mw {
@@ -284,8 +295,14 @@ static int ntb_async_tx_submit(struct ntb_transport_qp *qp,
static void ntb_memcpy_tx(struct ntb_queue_entry *entry, void __iomem *offset);
static int ntb_async_rx_submit(struct ntb_queue_entry *entry, void *offset);
static void ntb_memcpy_rx(struct ntb_queue_entry *entry, void *offset);
static int ntb_tx_memcpy_kthread(void *data);
static inline bool ntb_tx_offload_enabled(struct ntb_transport_qp *qp)
{
return tx_memcpy_offload && qp && qp->tx_offload_thread;
}
static int ntb_transport_bus_match(struct device *dev,
const struct device_driver *drv)
{
@@ -1254,11 +1271,13 @@ static int ntb_transport_init_queue(struct ntb_transport_ctx *nt,
spin_lock_init(&qp->ntb_rx_q_lock);
spin_lock_init(&qp->ntb_tx_free_q_lock);
spin_lock_init(&qp->ntb_tx_offl_q_lock);
INIT_LIST_HEAD(&qp->rx_post_q);
INIT_LIST_HEAD(&qp->rx_pend_q);
INIT_LIST_HEAD(&qp->rx_free_q);
INIT_LIST_HEAD(&qp->tx_free_q);
INIT_LIST_HEAD(&qp->tx_offl_q);
tasklet_init(&qp->rxc_db_work, ntb_transport_rxc_db,
(unsigned long)qp);
@@ -1785,6 +1804,13 @@ static void ntb_tx_copy_callback(void *data,
iowrite32(entry->flags | DESC_DONE_FLAG, &hdr->flags);
/*
* Make DONE flag visible before DB/MSI. WC + posted MWr may reorder
* across iATU/bridge (platform-dependent). Order and flush here.
*/
dma_mb();
ioread32(&hdr->flags);
if (qp->use_msi)
ntb_msi_peer_trigger(qp->ndev, PIDX, &qp->peer_msi_desc);
else
@@ -1805,7 +1831,7 @@ static void ntb_tx_copy_callback(void *data,
ntb_list_add(&qp->ntb_tx_free_q_lock, &entry->entry, &qp->tx_free_q);
}
static void ntb_memcpy_tx(struct ntb_queue_entry *entry, void __iomem *offset)
static void ntb_memcpy_tx_on_stack(struct ntb_queue_entry *entry, void __iomem *offset)
{
#ifdef ARCH_HAS_NOCACHE_UACCESS
/*
@@ -1823,6 +1849,54 @@ static void ntb_memcpy_tx(struct ntb_queue_entry *entry, void __iomem *offset)
ntb_tx_copy_callback(entry, NULL);
}
static int ntb_tx_memcpy_kthread(void *data)
{
struct ntb_transport_qp *qp = data;
struct ntb_queue_entry *entry, *tmp;
const int resched_nr = 64;
LIST_HEAD(local_list);
void __iomem *offset;
int processed = 0;
while (!kthread_should_stop()) {
spin_lock_irq(&qp->ntb_tx_offl_q_lock);
wait_event_interruptible_lock_irq_timeout(qp->tx_offload_wq,
kthread_should_stop() ||
!list_empty(&qp->tx_offl_q),
qp->ntb_tx_offl_q_lock, 5*HZ);
list_splice_tail_init(&qp->tx_offl_q, &local_list);
spin_unlock_irq(&qp->ntb_tx_offl_q_lock);
list_for_each_entry_safe(entry, tmp, &local_list, entry) {
list_del(&entry->entry);
offset = qp->tx_mw + qp->tx_max_frame * entry->tx_index;
ntb_memcpy_tx_on_stack(entry, offset);
if (++processed >= resched_nr) {
cond_resched();
processed = 0;
}
}
cond_resched();
}
return 0;
}
static void ntb_memcpy_tx(struct ntb_queue_entry *entry, void __iomem *offset)
{
struct ntb_transport_qp *qp = entry->qp;
if (WARN_ON_ONCE(!qp))
return;
if (ntb_tx_offload_enabled(qp)) {
ntb_list_add(&qp->ntb_tx_offl_q_lock, &entry->entry,
&qp->tx_offl_q);
wake_up(&qp->tx_offload_wq);
} else
ntb_memcpy_tx_on_stack(entry, offset);
}
static int ntb_async_tx_submit(struct ntb_transport_qp *qp,
struct ntb_queue_entry *entry)
{
@@ -1895,6 +1969,9 @@ static void ntb_async_tx(struct ntb_transport_qp *qp,
hdr = offset + qp->tx_max_frame - sizeof(struct ntb_payload_header);
entry->tx_hdr = hdr;
WARN_ON_ONCE(!ntb_transport_tx_free_entry(qp));
WRITE_ONCE(qp->tx_index, (qp->tx_index + 1) % qp->tx_max_entry);
iowrite32(entry->len, &hdr->len);
iowrite32((u32)qp->tx_pkts, &hdr->ver);
@@ -1935,9 +2012,6 @@ static int ntb_process_tx(struct ntb_transport_qp *qp,
ntb_async_tx(qp, entry);
qp->tx_index++;
qp->tx_index %= qp->tx_max_entry;
qp->tx_pkts++;
return 0;
@@ -2034,6 +2108,20 @@ ntb_transport_create_queue(void *data, struct device *client_dev,
qp->tx_handler = handlers->tx_handler;
qp->event_handler = handlers->event_handler;
init_waitqueue_head(&qp->tx_offload_wq);
if (tx_memcpy_offload) {
qp->tx_offload_thread = kthread_run(ntb_tx_memcpy_kthread, qp,
"ntb-txcpy/%s/%u",
pci_name(ndev->pdev), qp->qp_num);
if (IS_ERR(qp->tx_offload_thread)) {
dev_warn(&nt->ndev->dev,
"tx memcpy offload thread creation failed: %ld; falling back to inline copy\n",
PTR_ERR(qp->tx_offload_thread));
qp->tx_offload_thread = NULL;
}
} else
qp->tx_offload_thread = NULL;
dma_cap_zero(dma_mask);
dma_cap_set(DMA_MEMCPY, dma_mask);
@@ -2141,6 +2229,11 @@ void ntb_transport_free_queue(struct ntb_transport_qp *qp)
qp->active = false;
if (qp->tx_offload_thread) {
kthread_stop(qp->tx_offload_thread);
qp->tx_offload_thread = NULL;
}
if (qp->tx_dma_chan) {
struct dma_chan *chan = qp->tx_dma_chan;
/* Putting the dma_chan to NULL will force any new traffic to be
@@ -2204,6 +2297,9 @@ void ntb_transport_free_queue(struct ntb_transport_qp *qp)
while ((entry = ntb_list_rm(&qp->ntb_tx_free_q_lock, &qp->tx_free_q)))
kfree(entry);
while ((entry = ntb_list_rm(&qp->ntb_tx_offl_q_lock, &qp->tx_offl_q)))
kfree(entry);
qp->transport->qp_bitmap_free |= qp_bit;
dev_info(&pdev->dev, "NTB Transport QP %d freed\n", qp->qp_num);