apple-nvme: defer cache flushes by a specified amount

Cache flushes on the M1 nvme are really slow, taking 17-18 msec to
complete. This can slow down workloads considerably, pure random writes
end up being bound by the flush latency and hence run at 55-60 IOPS.

Add a deferred flush work around to provide better performance, at a
minimal risk. By default, flushes are delayed at most 1 second, but this
is configurable.

With this work-around, a pure random write workload runs at ~12K IOPS
rather than 56 IOPS.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

drivers/nvme/host/apple.c

@@ -195,8 +195,20 @@ struct apple_nvme {
 
 	int irq;
 	spinlock_t lock;
+
+	/*
+	 * Delayed cache flush handling state
+	 */
+	struct nvme_ns *flush_ns;
+	unsigned long flush_interval;
+	unsigned long last_flush;
+	struct delayed_work flush_dwork;
 };
 
+unsigned int flush_interval = 1000;
+module_param(flush_interval, uint, 0644);
+MODULE_PARM_DESC(flush_interval, "Grace period in msecs between flushes");
+
 static_assert(sizeof(struct nvme_command) == 64);
 static_assert(sizeof(struct apple_nvmmu_tcb) == 128);
 
@@ -729,6 +741,26 @@ static int apple_nvme_remove_sq(struct apple_nvme *anv)
 	return nvme_submit_sync_cmd(anv->ctrl.admin_q, &c, NULL, 0);
 }
 
+static bool apple_nvme_delayed_flush(struct apple_nvme *anv, struct nvme_ns *ns,
+				     struct request *req)
+{
+	if (!anv->flush_interval || req_op(req) != REQ_OP_FLUSH)
+		return false;
+	if (delayed_work_pending(&anv->flush_dwork))
+		return true;
+	if (time_before(jiffies, anv->last_flush + anv->flush_interval)) {
+		kblockd_mod_delayed_work_on(WORK_CPU_UNBOUND, &anv->flush_dwork,
+						anv->flush_interval);
+		if (WARN_ON_ONCE(anv->flush_ns && anv->flush_ns != ns))
+			goto out;
+		anv->flush_ns = ns;
+		return true;
+	}
+out:
+	anv->last_flush = jiffies;
+	return false;
+}
+
 static blk_status_t apple_nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
 					const struct blk_mq_queue_data *bd)
 {
@@ -764,6 +796,12 @@ static blk_status_t apple_nvme_queue_rq(struct blk_mq_hw_ctx *hctx,
 	}
 
 	nvme_start_request(req);
+
+	if (apple_nvme_delayed_flush(anv, ns, req)) {
+		blk_mq_complete_request(req);
+		return BLK_STS_OK;
+	}
+
 	apple_nvme_submit_cmd(q, cmnd);
 	return BLK_STS_OK;
 
@@ -1388,6 +1426,28 @@ static void devm_apple_nvme_mempool_destroy(void *data)
 	mempool_destroy(data);
 }
 
+static void apple_nvme_flush_work(struct work_struct *work)
+{
+	struct nvme_command c = { };
+	struct apple_nvme *anv;
+	struct nvme_ns *ns;
+	int err;
+
+	anv = container_of(work, struct apple_nvme, flush_dwork.work);
+	ns = anv->flush_ns;
+	if (WARN_ON_ONCE(!ns))
+		return;
+
+	c.common.opcode = nvme_cmd_flush;
+	c.common.nsid = cpu_to_le32(anv->flush_ns->head->ns_id);
+	err = nvme_submit_sync_cmd(ns->queue, &c, NULL, 0);
+	if (err) {
+		dev_err(anv->dev, "Deferred flush failed: %d\n", err);
+	} else {
+		anv->last_flush = jiffies;
+	}
+}
+
 static struct apple_nvme *apple_nvme_alloc(struct platform_device *pdev)
 {
 	struct device *dev = &pdev->dev;
@@ -1542,6 +1602,14 @@ static int apple_nvme_probe(struct platform_device *pdev)
 		goto out_uninit_ctrl;
 	}
 
+	if (flush_interval) {
+		anv->flush_interval = msecs_to_jiffies(flush_interval);
+		anv->flush_ns = NULL;
+		anv->last_flush = jiffies - anv->flush_interval;
+	}
+
+	INIT_DELAYED_WORK(&anv->flush_dwork, apple_nvme_flush_work);
+
 	nvme_reset_ctrl(&anv->ctrl);
 	async_schedule(apple_nvme_async_probe, anv);
 
@@ -1575,6 +1643,7 @@ static void apple_nvme_shutdown(struct platform_device *pdev)
 {
 	struct apple_nvme *anv = platform_get_drvdata(pdev);
 
+	flush_delayed_work(&anv->flush_dwork);
 	apple_nvme_disable(anv, true);
 	if (apple_rtkit_is_running(anv->rtk))
 		apple_rtkit_shutdown(anv->rtk);