Re: [PATCH v2] target: add iscsi/cpus_allowed_list in configfs

Mike Christie
2022-02-28T18:21:18Z

On 2/17/22 1:45 AM, mingzhe.zou@easystack.cn wrote:
> From: Mingzhe Zou <mingzhe.zou@easystack.cn>
>
> The RX/TX threads for iSCSI connection can be scheduled to
> any online cpus, and will not be rescheduled.
>
> If bind other heavy load threads with iSCSI connection
> RX/TX thread to the same cpu, the iSCSI performance will
> be worse.
>
> This patch add iscsi/cpus_allowed_list in configfs. The
> available cpus set of iSCSI connection RX/TX threads is
> allowed_cpus & online_cpus. If it is modified, all RX/TX
> threads will be rescheduled.
>
> Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>
> ---
> drivers/target/iscsi/iscsi_target.c | 66 +++++++++++++++++++-
> drivers/target/iscsi/iscsi_target_configfs.c | 32 ++++++++++
> drivers/target/iscsi/iscsi_target_login.c | 8 +++
> include/target/iscsi/iscsi_target_core.h | 31 ++-------
> 4 files changed, 109 insertions(+), 28 deletions(-)
>
> diff --git a/drivers/target/iscsi/iscsi_target.c b/drivers/target/iscsi/iscsi_target.c
> index 2c54c5d8412d..82f54b59996d 100644
> --- a/drivers/target/iscsi/iscsi_target.c
> +++ b/drivers/target/iscsi/iscsi_target.c
> @@ -702,13 +702,19 @@ static int __init iscsi_target_init_module(void)
> if (!iscsit_global->ts_bitmap)
> goto configfs_out;
>
> + if (!zalloc_cpumask_var(&iscsit_global->allowed_cpumask, GFP_KERNEL)) {
> + pr_err("Unable to allocate iscsit_global->allowed_cpumask\n");
> + goto bitmap_out;
> + }
> + cpumask_setall(iscsit_global->allowed_cpumask);
> +
> lio_qr_cache = kmem_cache_create("lio_qr_cache",
> sizeof(struct iscsi_queue_req),
> __alignof__(struct iscsi_queue_req), 0, NULL);
> if (!lio_qr_cache) {
> pr_err("Unable to kmem_cache_create() for"
> " lio_qr_cache\n");
> - goto bitmap_out;
> + goto cpumask_out;
> }
>
> lio_dr_cache = kmem_cache_create("lio_dr_cache",
> @@ -753,6 +759,8 @@ static int __init iscsi_target_init_module(void)
> kmem_cache_destroy(lio_dr_cache);
> qr_out:
> kmem_cache_destroy(lio_qr_cache);
> +cpumask_out:
> + free_cpumask_var(iscsit_global->allowed_cpumask);
> bitmap_out:
> vfree(iscsit_global->ts_bitmap);
> configfs_out:
> @@ -782,6 +790,7 @@ static void __exit iscsi_target_cleanup_module(void)
>
> target_unregister_template(&iscsi_ops);
>
> + free_cpumask_var(iscsit_global->allowed_cpumask);
> vfree(iscsit_global->ts_bitmap);
> kfree(iscsit_global);
> }
> @@ -3587,6 +3596,11 @@ static int iscsit_send_reject(
> void iscsit_thread_get_cpumask(struct iscsi_conn *conn)
> {
> int ord, cpu;
> + cpumask_t conn_allowed_cpumask;
> +
> + cpumask_and(&conn_allowed_cpumask, iscsit_global->allowed_cpumask,
> + cpu_online_mask);
> +
> /*
> * bitmap_id is assigned from iscsit_global->ts_bitmap from
> * within iscsit_start_kthreads()
> @@ -3595,8 +3609,9 @@ void iscsit_thread_get_cpumask(struct iscsi_conn *conn)
> * iSCSI connection's RX/TX threads will be scheduled to
> * execute upon.
> */
> - ord = conn->bitmap_id % cpumask_weight(cpu_online_mask);
> - for_each_online_cpu(cpu) {
> + cpumask_clear(conn->conn_cpumask);
> + ord = conn->bitmap_id % cpumask_weight(&conn_allowed_cpumask);
> + for_each_cpu(cpu, &conn_allowed_cpumask) {
> if (ord-- == 0) {
> cpumask_set_cpu(cpu, conn->conn_cpumask);
> return;
> @@ -3609,6 +3624,51 @@ void iscsit_thread_get_cpumask(struct iscsi_conn *conn)
> cpumask_setall(conn->conn_cpumask);
> }
>
> +static void iscsit_thread_reschedule(struct iscsi_conn *conn)
> +{
> + /*
> + * If iscsit_global->allowed_cpumask modified, reschedule iSCSI
> + * connection's RX/TX threads update conn->allowed_cpumask.
> + */
> + if (!cpumask_equal(iscsit_global->allowed_cpumask,
> + conn->allowed_cpumask)) {
> + iscsit_thread_get_cpumask(conn);
> + conn->conn_tx_reset_cpumask = 1;
> + conn->conn_rx_reset_cpumask = 1;
> + cpumask_copy(conn->allowed_cpumask,
> + iscsit_global->allowed_cpumask);
> + }
> +}
> +
> +void iscsit_thread_check_cpumask(
> + struct iscsi_conn *conn,
> + struct task_struct *p,
> + int mode)
> +{
> + iscsit_thread_reschedule(conn);
> +
> + /*
> + * mode == 1 signals iscsi_target_tx_thread() usage.
> + * mode == 0 signals iscsi_target_rx_thread() usage.
> + */
> + if (mode == 1) {
> + if (!conn->conn_tx_reset_cpumask)
> + return;
> + conn->conn_tx_reset_cpumask = 0;
> + } else {
> + if (!conn->conn_rx_reset_cpumask)
> + return;
> + conn->conn_rx_reset_cpumask = 0;
> + }
> + /*
> + * Update the CPU mask for this single kthread so that
> + * both TX and RX kthreads are scheduled to run on the
> + * same CPU.
> + */
> + set_cpus_allowed_ptr(p, conn->conn_cpumask);
> +}

We can hit a race where we call this twice for the same CPU right?

The rx and tx thread both call iscsit_thread_reschedule and cpumask_equal
and it returns false for them. The rx thread might be faster and return
from iscsit_thread_reschedule and is setting conn_rx_reset_cpumask to 0.
Then the tx thread is sets it back to 1. The next time the rx thread loops
it sees conn_rx_reset_cpumask set to 1 and calls set_cpus_allowed_ptr.

Is that the only possible race? If so it seems ok. Maybe just add a comment,
so later when someone else is looking at the code they don't waste time
and think it's broken and know it was intentional or at least we didn't care.