scsi: use percpu counters for iorequest_cnt and iodone_cnt

iorequest_cnt and iodone_cnt are updated on every command dispatch and
completion, often from different CPUs on high queue depth workloads.
Using adjacent atomic_t fields caused cache line contention between the
submission and completion paths.

Represent these statistics with struct percpu_counter so increments are
mostly local to each CPU, avoiding false sharing without growing
struct scsi_device further for cache-line padding.

Suggested-by: Bart Van Assche <[email protected]>
Signed-off-by: Sumit Saxena <[email protected]>

Author: sumitsaxena11

drivers/scsi/scsi_error.c

@@ -370,7 +370,7 @@ enum blk_eh_timer_return scsi_timeout(struct request *req)
 	 */
 	if (test_and_set_bit(SCMD_STATE_COMPLETE, &scmd->state))
 		return BLK_EH_DONE;
-	atomic_inc(&scmd->device->iodone_cnt);
+	percpu_counter_inc(&scmd->device->iodone_cnt);
 	if (scsi_abort_command(scmd) != SUCCESS) {
 		set_host_byte(scmd, DID_TIME_OUT);
 		scsi_eh_scmd_add(scmd);

drivers/scsi/scsi_lib.c

@@ -1553,7 +1553,7 @@ static void scsi_complete(struct request *rq)
 
 	INIT_LIST_HEAD(&cmd->eh_entry);
 
-	atomic_inc(&cmd->device->iodone_cnt);
+	percpu_counter_inc(&cmd->device->iodone_cnt);
 	if (cmd->result)
 		atomic_inc(&cmd->device->ioerr_cnt);
 
@@ -1591,7 +1591,7 @@ static enum scsi_qc_status scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 	struct Scsi_Host *host = cmd->device->host;
 	int rtn = 0;
 
-	atomic_inc(&cmd->device->iorequest_cnt);
+	percpu_counter_inc(&cmd->device->iorequest_cnt);
 
 	/* check if the device is still usable */
 	if (unlikely(cmd->device->sdev_state == SDEV_DEL)) {
@@ -1613,7 +1613,7 @@ static enum scsi_qc_status scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 		 */
 		SCSI_LOG_MLQUEUE(3, scmd_printk(KERN_INFO, cmd,
 			"queuecommand : device blocked\n"));
-		atomic_dec(&cmd->device->iorequest_cnt);
+		percpu_counter_dec(&cmd->device->iorequest_cnt);
 		return SCSI_MLQUEUE_DEVICE_BUSY;
 	}
 
@@ -1646,7 +1646,7 @@ static enum scsi_qc_status scsi_dispatch_cmd(struct scsi_cmnd *cmd)
 	trace_scsi_dispatch_cmd_start(cmd);
 	rtn = host->hostt->queuecommand(host, cmd);
 	if (rtn) {
-		atomic_dec(&cmd->device->iorequest_cnt);
+		percpu_counter_dec(&cmd->device->iorequest_cnt);
 		trace_scsi_dispatch_cmd_error(cmd, rtn);
 		if (rtn != SCSI_MLQUEUE_DEVICE_BUSY &&
 		    rtn != SCSI_MLQUEUE_TARGET_BUSY)

drivers/scsi/scsi_scan.c

@@ -351,6 +351,15 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 
 	scsi_sysfs_device_initialize(sdev);
 
+	ret = percpu_counter_init(&sdev->iorequest_cnt, 0, GFP_KERNEL);
+	if (ret)
+		goto out_device_destroy;
+	ret = percpu_counter_init(&sdev->iodone_cnt, 0, GFP_KERNEL);
+	if (ret) {
+		percpu_counter_destroy(&sdev->iorequest_cnt);
+		goto out_device_destroy;
+	}
+
 	if (scsi_device_is_pseudo_dev(sdev))
 		return sdev;
 

drivers/scsi/scsi_sysfs.c

@@ -516,6 +516,10 @@ static void scsi_device_dev_release(struct device *dev)
 	if (vpd_pgb7)
 		kfree_rcu(vpd_pgb7, rcu);
 	kfree(sdev->inquiry);
+	if (percpu_counter_initialized(&sdev->iodone_cnt))
+		percpu_counter_destroy(&sdev->iodone_cnt);
+	if (percpu_counter_initialized(&sdev->iorequest_cnt))
+		percpu_counter_destroy(&sdev->iorequest_cnt);
 	kfree(sdev);
 
 	if (parent)
@@ -936,11 +940,26 @@ static ssize_t
 show_iostat_counterbits(struct device *dev, struct device_attribute *attr,
 			char *buf)
 {
-	return snprintf(buf, 20, "%d\n", (int)sizeof(atomic_t) * 8);
+	/*
+	 * iorequest_cnt and iodone_cnt are per-CPU sums (s64); ioerr_cnt and
+	 * iotmo_cnt remain atomic_t.  Report the widest counter for tools.
+	 */
+	return snprintf(buf, 20, "%zu\n", sizeof(s64) * 8);
 }
 
 static DEVICE_ATTR(iocounterbits, S_IRUGO, show_iostat_counterbits, NULL);
 
+#define show_sdev_iostat_percpu(field)					\
+static ssize_t								\
+show_iostat_##field(struct device *dev, struct device_attribute *attr,	\
+		    char *buf)						\
+{									\
+	struct scsi_device *sdev = to_scsi_device(dev);			\
+	unsigned long long count = percpu_counter_sum(&sdev->field);	\
+	return snprintf(buf, 20, "0x%llx\n", count);			\
+}									\
+static DEVICE_ATTR(field, 0444, show_iostat_##field, NULL)
+
 #define show_sdev_iostat(field)						\
 static ssize_t								\
 show_iostat_##field(struct device *dev, struct device_attribute *attr,	\
@@ -950,10 +969,10 @@ show_iostat_##field(struct device *dev, struct device_attribute *attr,	\
 	unsigned long long count = atomic_read(&sdev->field);		\
 	return snprintf(buf, 20, "0x%llx\n", count);			\
 }									\
-static DEVICE_ATTR(field, S_IRUGO, show_iostat_##field, NULL)
+static DEVICE_ATTR(field, 0444, show_iostat_##field, NULL)
 
-show_sdev_iostat(iorequest_cnt);
-show_sdev_iostat(iodone_cnt);
+show_sdev_iostat_percpu(iorequest_cnt);
+show_sdev_iostat_percpu(iodone_cnt);
 show_sdev_iostat(ioerr_cnt);
 show_sdev_iostat(iotmo_cnt);
 

include/scsi/scsi_device.h

@@ -8,6 +8,7 @@
 #include <linux/blk-mq.h>
 #include <scsi/scsi.h>
 #include <linux/atomic.h>
+#include <linux/percpu_counter.h>
 #include <linux/sbitmap.h>
 
 struct bsg_device;
@@ -271,8 +272,8 @@ struct scsi_device {
 	unsigned int max_device_blocked; /* what device_blocked counts down from  */
 #define SCSI_DEFAULT_DEVICE_BLOCKED	3
 
-	atomic_t iorequest_cnt;
-	atomic_t iodone_cnt;
+	struct percpu_counter iorequest_cnt;
+	struct percpu_counter iodone_cnt;
 	atomic_t ioerr_cnt;
 	atomic_t iotmo_cnt;