On 01/09/2019 12:23 PM, Waiman Long wrote:
> After skipping the percpu summation of non-active IRQs on a 4-socket
> Broadwell system with about 3k IRQs, about half of the CPU cycles were
> spent in the kstat_irqs() call. The majority of which were used to look
> up the IRQ descriptors for the corresponding IRQ numbers.
>
> We can recoup a lot of those lost cycles by calling kstat_irqs_usr()
> only for those IRQs that are active. A bitmap is now used to keep track
> of the list of the active IRQs. Changes in nr_active_irqs count will
> cause the code to rescan all the IRQs and repopulate the bitmap.
>
> On the same 4-socket server, the introduction of this patch further
> reduces the system time of reading /proc/stat 5k times from 8.048s
> to 5.817s. This is a another time reduction of 28%.
>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> fs/proc/stat.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 84 insertions(+)
>
> diff --git a/fs/proc/stat.c b/fs/proc/stat.c
> index 4b06f1b..5e2a398 100644
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -93,6 +93,25 @@ static u64 compute_stat_irqs_sum(void)
> }
>
> /*
> + * Write the given number of space separated '0' into the sequence file.
> + */
> +static void write_zeros(struct seq_file *p, int cnt)
> +{
> + /* String of 16 '0's */
> + static const char zeros[] = " 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0";
> +
> + while (cnt > 0) {
> + if (cnt >= 16) {
> + seq_write(p, zeros, 32);
> + cnt -= 16;
> + } else {
> + seq_write(p, zeros, 2 * cnt);
> + cnt = 0;
> + }
> + }
> +}
> +
> +/*
> * Print out the "intr" line of /proc/stat.
> */
> static void show_stat_irqs(struct seq_file *p)
> @@ -100,9 +119,74 @@ static void show_stat_irqs(struct seq_file *p)
> int i;
>
> seq_put_decimal_ull(p, "intr ", compute_stat_irqs_sum());
> +
> + if (IS_ENABLED(CONFIG_SMP) && (nr_cpu_ids >= 10) && (nr_irqs >= 256)) {
> + /*
> + * On systems with 10 or more CPUs and 256 or more IRQs,
> + * we used a bitmap to keep track of the number of active
> + * IRQs and call kstat_irqs_usr() only for those IRQs.
> + * The bitmap will be refreshed whenever nr_active_irqs
> + * changes.
> + */
> + extern atomic_t nr_active_irqs;
> + static DEFINE_MUTEX(irqs_mutex);
> + static int last_irq = -1;
> + static int bitmap_size, active_irqs;
> + static unsigned long *bitmap;
> + int current_irqs = atomic_read(&nr_active_irqs);
> +
> + mutex_lock(&irqs_mutex);
> + if (current_irqs != active_irqs) {
> + /*
> + * Rescan all the IRQs for active ones.
> + */
> + if (nr_irqs > bitmap_size) {
> + static unsigned long *new_bitmap;
> + static int new_size;
> +
> + new_size = BITS_TO_LONGS(nr_irqs)*sizeof(long);
> + new_bitmap = (unsigned long *)krealloc(bitmap,
> + new_size, GFP_KERNEL);
> + if (!new_bitmap)
> + goto fallback;
> + bitmap = new_bitmap;
> + bitmap_size = new_size;
> + }
> + memset(bitmap, 0, bitmap_size/BITS_PER_BYTE);
> + last_irq = 0;
Sorry, last_irq should be initialized to -1 here.
Cheers,
Longman
