Re: [PATCH 2/2] kcompressd: Add Kcompressd for accelerated zram compression

[Barry Song <21cnbao@gmail.com>]

On Sat, Mar 8, 2025 at 1:02 AM Qun-Wei Lin <qun-wei.lin@mediatek.com> wrote:
>
> Introduced Kcompressd to offload zram page compression, improving
> system efficiency by handling compression separately from memory
> reclaiming. Added necessary configurations and dependencies.
>
> Signed-off-by: Qun-Wei Lin <qun-wei.lin@mediatek.com>
> ---
>  drivers/block/zram/Kconfig      |  11 ++
>  drivers/block/zram/Makefile     |   3 +-
>  drivers/block/zram/kcompressd.c | 340 ++++++++++++++++++++++++++++++++
>  drivers/block/zram/kcompressd.h |  25 +++
>  drivers/block/zram/zram_drv.c   |  22 ++-
>  5 files changed, 397 insertions(+), 4 deletions(-)
>  create mode 100644 drivers/block/zram/kcompressd.c
>  create mode 100644 drivers/block/zram/kcompressd.h
>
> diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
> index 402b7b175863..f0a1b574f770 100644
> --- a/drivers/block/zram/Kconfig
> +++ b/drivers/block/zram/Kconfig
> @@ -145,3 +145,14 @@ config ZRAM_MULTI_COMP
>           re-compress pages using a potentially slower but more effective
>           compression algorithm. Note, that IDLE page recompression
>           requires ZRAM_TRACK_ENTRY_ACTIME.
> +
> +config KCOMPRESSD
> +       tristate "Kcompressd: Accelerated zram compression"
> +       depends on ZRAM
> +       help
> +         Kcompressd creates multiple daemons to accelerate the compression of pages
> +         in zram, offloading this time-consuming task from the zram driver.
> +
> +         This approach improves system efficiency by handling page compression separately,
> +         which was originally done by kswapd or direct reclaim.

For direct reclaim, we were previously able to compress using multiple CPUs
with multi-threading.
After your patch, it seems that only a single thread/CPU is used for compression
so it won't necessarily improve direct reclaim performance?

Even for kswapd, we used to have multiple threads like [kswapd0], [kswapd1],
and [kswapd2] for different nodes. Now, are we also limited to just one thread?
I also wonder if this could be handled at the vmscan level instead of the zram
level. then it might potentially help other sync devices or even zswap later.

But I agree that for phones, modifying zram seems like an easier starting
point. However, relying on a single thread isn't always the best approach.


> +
> diff --git a/drivers/block/zram/Makefile b/drivers/block/zram/Makefile
> index 0fdefd576691..23baa5dfceb9 100644
> --- a/drivers/block/zram/Makefile
> +++ b/drivers/block/zram/Makefile
> @@ -9,4 +9,5 @@ zram-$(CONFIG_ZRAM_BACKEND_ZSTD)        += backend_zstd.o
>  zram-$(CONFIG_ZRAM_BACKEND_DEFLATE)    += backend_deflate.o
>  zram-$(CONFIG_ZRAM_BACKEND_842)                += backend_842.o
>
> -obj-$(CONFIG_ZRAM)     +=      zram.o
> +obj-$(CONFIG_ZRAM)             += zram.o
> +obj-$(CONFIG_KCOMPRESSD)       += kcompressd.o
> diff --git a/drivers/block/zram/kcompressd.c b/drivers/block/zram/kcompressd.c
> new file mode 100644
> index 000000000000..195b7e386869
> --- /dev/null
> +++ b/drivers/block/zram/kcompressd.c
> @@ -0,0 +1,340 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (C) 2024 MediaTek Inc.
> + */
> +
> +#include <linux/module.h>
> +#include <linux/kernel.h>
> +#include <linux/bio.h>
> +#include <linux/bitops.h>
> +#include <linux/freezer.h>
> +#include <linux/kernel.h>
> +#include <linux/psi.h>
> +#include <linux/kfifo.h>
> +#include <linux/swap.h>
> +#include <linux/delay.h>
> +
> +#include "kcompressd.h"
> +
> +#define INIT_QUEUE_SIZE                4096
> +#define DEFAULT_NR_KCOMPRESSD  4
> +
> +static atomic_t enable_kcompressd;
> +static unsigned int nr_kcompressd;
> +static unsigned int queue_size_per_kcompressd;
> +static struct kcompress *kcompress;
> +
> +enum run_state {
> +       KCOMPRESSD_NOT_STARTED = 0,
> +       KCOMPRESSD_RUNNING,
> +       KCOMPRESSD_SLEEPING,
> +};
> +
> +struct kcompressd_para {
> +       wait_queue_head_t *kcompressd_wait;
> +       struct kfifo *write_fifo;
> +       atomic_t *running;
> +};
> +
> +static struct kcompressd_para *kcompressd_para;
> +static BLOCKING_NOTIFIER_HEAD(kcompressd_notifier_list);
> +
> +struct write_work {
> +       void *mem;
> +       struct bio *bio;
> +       compress_callback cb;
> +};
> +
> +int kcompressd_enabled(void)
> +{
> +       return likely(atomic_read(&enable_kcompressd));
> +}
> +EXPORT_SYMBOL(kcompressd_enabled);
> +
> +static void kcompressd_try_to_sleep(struct kcompressd_para *p)
> +{
> +       DEFINE_WAIT(wait);
> +
> +       if (!kfifo_is_empty(p->write_fifo))
> +               return;
> +
> +       if (freezing(current) || kthread_should_stop())
> +               return;
> +
> +       atomic_set(p->running, KCOMPRESSD_SLEEPING);
> +       prepare_to_wait(p->kcompressd_wait, &wait, TASK_INTERRUPTIBLE);
> +
> +       /*
> +        * After a short sleep, check if it was a premature sleep. If not, then
> +        * go fully to sleep until explicitly woken up.
> +        */
> +       if (!kthread_should_stop() && kfifo_is_empty(p->write_fifo))
> +               schedule();
> +
> +       finish_wait(p->kcompressd_wait, &wait);
> +       atomic_set(p->running, KCOMPRESSD_RUNNING);
> +}
> +
> +static int kcompressd(void *para)
> +{
> +       struct task_struct *tsk = current;
> +       struct kcompressd_para *p = (struct kcompressd_para *)para;
> +
> +       tsk->flags |= PF_MEMALLOC | PF_KSWAPD;
> +       set_freezable();
> +
> +       while (!kthread_should_stop()) {
> +               bool ret;
> +
> +               kcompressd_try_to_sleep(p);
> +               ret = try_to_freeze();
> +               if (kthread_should_stop())
> +                       break;
> +
> +               if (ret)
> +                       continue;
> +
> +               while (!kfifo_is_empty(p->write_fifo)) {
> +                       struct write_work entry;
> +
> +                       if (sizeof(struct write_work) == kfifo_out(p->write_fifo,
> +                                               &entry, sizeof(struct write_work))) {
> +                               entry.cb(entry.mem, entry.bio);
> +                               bio_put(entry.bio);
> +                       }
> +               }
> +
> +       }
> +
> +       tsk->flags &= ~(PF_MEMALLOC | PF_KSWAPD);
> +       atomic_set(p->running, KCOMPRESSD_NOT_STARTED);
> +       return 0;
> +}
> +
> +static int init_write_queue(void)
> +{
> +       int i;
> +       unsigned int queue_len = queue_size_per_kcompressd * sizeof(struct write_work);
> +
> +       for (i = 0; i < nr_kcompressd; i++) {
> +               if (kfifo_alloc(&kcompress[i].write_fifo,
> +                                       queue_len, GFP_KERNEL)) {
> +                       pr_err("Failed to alloc kfifo %d\n", i);
> +                       return -ENOMEM;
> +               }
> +       }
> +       return 0;
> +}
> +
> +static void clean_bio_queue(int idx)
> +{
> +       struct write_work entry;
> +
> +       while (sizeof(struct write_work) == kfifo_out(&kcompress[idx].write_fifo,
> +                               &entry, sizeof(struct write_work))) {
> +               bio_put(entry.bio);
> +               entry.cb(entry.mem, entry.bio);
> +       }
> +       kfifo_free(&kcompress[idx].write_fifo);
> +}
> +
> +static int kcompress_update(void)
> +{
> +       int i;
> +       int ret;
> +
> +       kcompress = kvmalloc_array(nr_kcompressd, sizeof(struct kcompress), GFP_KERNEL);
> +       if (!kcompress)
> +               return -ENOMEM;
> +
> +       kcompressd_para = kvmalloc_array(nr_kcompressd, sizeof(struct kcompressd_para), GFP_KERNEL);
> +       if (!kcompressd_para)
> +               return -ENOMEM;
> +
> +       ret = init_write_queue();
> +       if (ret) {
> +               pr_err("Initialization of writing to FIFOs failed!!\n");
> +               return ret;
> +       }
> +
> +       for (i = 0; i < nr_kcompressd; i++) {
> +               init_waitqueue_head(&kcompress[i].kcompressd_wait);
> +               kcompressd_para[i].kcompressd_wait = &kcompress[i].kcompressd_wait;
> +               kcompressd_para[i].write_fifo = &kcompress[i].write_fifo;
> +               kcompressd_para[i].running = &kcompress[i].running;
> +       }
> +
> +       return 0;
> +}
> +
> +static void stop_all_kcompressd_thread(void)
> +{
> +       int i;
> +
> +       for (i = 0; i < nr_kcompressd; i++) {
> +               kthread_stop(kcompress[i].kcompressd);
> +               kcompress[i].kcompressd = NULL;
> +               clean_bio_queue(i);
> +       }
> +}
> +
> +static int do_nr_kcompressd_handler(const char *val,
> +               const struct kernel_param *kp)
> +{
> +       int ret;
> +
> +       atomic_set(&enable_kcompressd, false);
> +
> +       stop_all_kcompressd_thread();
> +
> +       ret = param_set_int(val, kp);
> +       if (!ret) {
> +               pr_err("Invalid number of kcompressd.\n");
> +               return -EINVAL;
> +       }
> +
> +       ret = init_write_queue();
> +       if (ret) {
> +               pr_err("Initialization of writing to FIFOs failed!!\n");
> +               return ret;
> +       }
> +
> +       atomic_set(&enable_kcompressd, true);
> +
> +       return 0;
> +}
> +
> +static const struct kernel_param_ops param_ops_change_nr_kcompressd = {
> +       .set = &do_nr_kcompressd_handler,
> +       .get = &param_get_uint,
> +       .free = NULL,
> +};
> +
> +module_param_cb(nr_kcompressd, &param_ops_change_nr_kcompressd,
> +               &nr_kcompressd, 0644);
> +MODULE_PARM_DESC(nr_kcompressd, "Number of pre-created daemon for page compression");
> +
> +static int do_queue_size_per_kcompressd_handler(const char *val,
> +               const struct kernel_param *kp)
> +{
> +       int ret;
> +
> +       atomic_set(&enable_kcompressd, false);
> +
> +       stop_all_kcompressd_thread();
> +
> +       ret = param_set_int(val, kp);
> +       if (!ret) {
> +               pr_err("Invalid queue size for kcompressd.\n");
> +               return -EINVAL;
> +       }
> +
> +       ret = init_write_queue();
> +       if (ret) {
> +               pr_err("Initialization of writing to FIFOs failed!!\n");
> +               return ret;
> +       }
> +
> +       pr_info("Queue size for kcompressd was changed: %d\n", queue_size_per_kcompressd);
> +
> +       atomic_set(&enable_kcompressd, true);
> +       return 0;
> +}
> +
> +static const struct kernel_param_ops param_ops_change_queue_size_per_kcompressd = {
> +       .set = &do_queue_size_per_kcompressd_handler,
> +       .get = &param_get_uint,
> +       .free = NULL,
> +};
> +
> +module_param_cb(queue_size_per_kcompressd, &param_ops_change_queue_size_per_kcompressd,
> +               &queue_size_per_kcompressd, 0644);
> +MODULE_PARM_DESC(queue_size_per_kcompressd,
> +               "Size of queue for kcompressd");
> +
> +int schedule_bio_write(void *mem, struct bio *bio, compress_callback cb)
> +{
> +       int i;
> +       bool submit_success = false;
> +       size_t sz_work = sizeof(struct write_work);
> +
> +       struct write_work entry = {
> +               .mem = mem,
> +               .bio = bio,
> +               .cb = cb
> +       };
> +
> +       if (unlikely(!atomic_read(&enable_kcompressd)))
> +               return -EBUSY;
> +
> +       if (!nr_kcompressd || !current_is_kswapd())
> +               return -EBUSY;
> +
> +       bio_get(bio);
> +
> +       for (i = 0; i < nr_kcompressd; i++) {
> +               submit_success =
> +                       (kfifo_avail(&kcompress[i].write_fifo) >= sz_work) &&
> +                       (sz_work == kfifo_in(&kcompress[i].write_fifo, &entry, sz_work));
> +
> +               if (submit_success) {
> +                       switch (atomic_read(&kcompress[i].running)) {
> +                       case KCOMPRESSD_NOT_STARTED:
> +                               atomic_set(&kcompress[i].running, KCOMPRESSD_RUNNING);
> +                               kcompress[i].kcompressd = kthread_run(kcompressd,
> +                                               &kcompressd_para[i], "kcompressd:%d", i);
> +                               if (IS_ERR(kcompress[i].kcompressd)) {
> +                                       atomic_set(&kcompress[i].running, KCOMPRESSD_NOT_STARTED);
> +                                       pr_warn("Failed to start kcompressd:%d\n", i);
> +                                       clean_bio_queue(i);
> +                               }
> +                               break;
> +                       case KCOMPRESSD_RUNNING:
> +                               break;
> +                       case KCOMPRESSD_SLEEPING:
> +                               wake_up_interruptible(&kcompress[i].kcompressd_wait);
> +                               break;
> +                       }
> +                       return 0;
> +               }
> +       }
> +
> +       bio_put(bio);
> +       return -EBUSY;
> +}
> +EXPORT_SYMBOL(schedule_bio_write);
> +
> +static int __init kcompressd_init(void)
> +{
> +       int ret;
> +
> +       nr_kcompressd = DEFAULT_NR_KCOMPRESSD;
> +       queue_size_per_kcompressd = INIT_QUEUE_SIZE;
> +
> +       ret = kcompress_update();
> +       if (ret) {
> +               pr_err("Init kcompressd failed!\n");
> +               return ret;
> +       }
> +
> +       atomic_set(&enable_kcompressd, true);
> +       blocking_notifier_call_chain(&kcompressd_notifier_list, 0, NULL);
> +       return 0;
> +}
> +
> +static void __exit kcompressd_exit(void)
> +{
> +       atomic_set(&enable_kcompressd, false);
> +       stop_all_kcompressd_thread();
> +
> +       kvfree(kcompress);
> +       kvfree(kcompressd_para);
> +}
> +
> +module_init(kcompressd_init);
> +module_exit(kcompressd_exit);
> +
> +MODULE_LICENSE("Dual BSD/GPL");
> +MODULE_AUTHOR("Qun-Wei Lin <qun-wei.lin@mediatek.com>");
> +MODULE_DESCRIPTION("Separate the page compression from the memory reclaiming");
> +
> diff --git a/drivers/block/zram/kcompressd.h b/drivers/block/zram/kcompressd.h
> new file mode 100644
> index 000000000000..2fe0b424a7af
> --- /dev/null
> +++ b/drivers/block/zram/kcompressd.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright (C) 2024 MediaTek Inc.
> + */
> +
> +#ifndef _KCOMPRESSD_H_
> +#define _KCOMPRESSD_H_
> +
> +#include <linux/rwsem.h>
> +#include <linux/kfifo.h>
> +#include <linux/atomic.h>
> +
> +typedef void (*compress_callback)(void *mem, struct bio *bio);
> +
> +struct kcompress {
> +       struct task_struct *kcompressd;
> +       wait_queue_head_t kcompressd_wait;
> +       struct kfifo write_fifo;
> +       atomic_t running;
> +};
> +
> +int kcompressd_enabled(void);
> +int schedule_bio_write(void *mem, struct bio *bio, compress_callback cb);
> +#endif
> +
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 2e1a70f2f4bd..bcd63ecb6ff2 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -35,6 +35,7 @@
>  #include <linux/part_stat.h>
>  #include <linux/kernel_read_file.h>
>
> +#include "kcompressd.h"
>  #include "zram_drv.h"
>
>  static DEFINE_IDR(zram_index_idr);
> @@ -2240,6 +2241,15 @@ static void zram_bio_write(struct zram *zram, struct bio *bio)
>         bio_endio(bio);
>  }
>
> +#if IS_ENABLED(CONFIG_KCOMPRESSD)
> +static void zram_bio_write_callback(void *mem, struct bio *bio)
> +{
> +       struct zram *zram = (struct zram *)mem;
> +
> +       zram_bio_write(zram, bio);
> +}
> +#endif
> +
>  /*
>   * Handler function for all zram I/O requests.
>   */
> @@ -2252,6 +2262,10 @@ static void zram_submit_bio(struct bio *bio)
>                 zram_bio_read(zram, bio);
>                 break;
>         case REQ_OP_WRITE:
> +#if IS_ENABLED(CONFIG_KCOMPRESSD)
> +               if (kcompressd_enabled() && !schedule_bio_write(zram, bio, zram_bio_write_callback))
> +                       break;
> +#endif
>                 zram_bio_write(zram, bio);
>                 break;
>         case REQ_OP_DISCARD:
> @@ -2535,9 +2549,11 @@ static int zram_add(void)
>  #if ZRAM_LOGICAL_BLOCK_SIZE == PAGE_SIZE
>                 .max_write_zeroes_sectors       = UINT_MAX,
>  #endif
> -               .features                       = BLK_FEAT_STABLE_WRITES        |
> -                                                 BLK_FEAT_READ_SYNCHRONOUS     |
> -                                                 BLK_FEAT_WRITE_SYNCHRONOUS,
> +               .features                       = BLK_FEAT_STABLE_WRITES
> +                                                 | BLK_FEAT_READ_SYNCHRONOUS
> +#if !IS_ENABLED(CONFIG_KCOMPRESSD)
> +                                                 | BLK_FEAT_WRITE_SYNCHRONOUS,
> +#endif
>         };
>         struct zram *zram;
>         int ret, device_id;
> --
> 2.45.2
>

Thanks
Barry