Re: [PATCH 1/2] mm/damon: introduce damon_rand_fast() for per-ctx PRNG

SeongJae Park
2026-04-24T01:36:30Z

Hello Jiayuan,

Thank you for sharing this patch with us!

On Thu, 23 Apr 2026 20:23:36 +0800 Jiayuan Chen <jiayuan.chen@linux.dev> wrote:

> From: Jiayuan Chen <jiayuan.chen@shopee.com>
>
> damon_rand() on the sampling_addr hot path calls get_random_u32_below(),
> which takes a local_lock_irqsave() around a per-CPU batched entropy pool
> and periodically refills it with ChaCha20. On workloads with large
> nr_regions (20k+), this shows up as a large fraction of kdamond CPU
> time: the lock_acquire / local_lock pair plus __get_random_u32_below()
> dominate perf profiles.

Could you please share more details about the use case? I'm particularly
curious how you ended up setting 'nr_regiions' that high, while the upper limit
of nr_regions is set to 1,0000 by default.

I know some people worry if the limit is too low and it could result in poor
monitoring accuracy. Therefore we developed [1] monitoring intervals
auto-tuning. From multiple tests on real environment it showed somewhat
convincing results, and therefore I nowadays suggest DAMON users to try that if
they didn't try.

I'm bit concerned if this is over-engineering. It would be helpful to know if
it is, if you could share the more detailed use case.

>
> Introduce damon_rand_fast(), which uses a lockless lfsr113 generator
> (struct rnd_state) held per damon_ctx and seeded from get_random_u64()
> in damon_new_ctx(). kdamond is the sole consumer of a given ctx, so no
> synchronization is required. Range mapping uses Lemire's
> (u64)rnd * span >> 32 to avoid a 64-bit division; residual bias is
> bounded by span / 2^32, negligible for statistical sampling.
>
> The new helper is intended for the sampling-address hot path only.
> damon_rand() is kept for call sites that run outside the kdamond loop
> and/or have no ctx available (damon_split_regions_of(), kunit tests).
>
> Convert the two hot callers:
>
> - __damon_pa_prepare_access_check()
> - __damon_va_prepare_access_check()
>
> lfsr113 is a linear PRNG and MUST NOT be used for anything
> security-sensitive. DAMON's sampling_addr is not exposed to userspace
> and is only consumed as a probe point for PTE accessed-bit sampling, so
> a non-cryptographic PRNG is appropriate here.
>
> Tested with paddr monitoring and max_nr_regions=20000: kdamond CPU
> usage reduced from ~72% to ~50% of one core.
>
> Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
> ---
> include/linux/damon.h | 28 ++++++++++++++++++++++++++++
> mm/damon/core.c | 2 ++
> mm/damon/paddr.c | 10 +++++-----
> mm/damon/vaddr.c | 9 +++++----
> 4 files changed, 40 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/damon.h b/include/linux/damon.h
> index f2cdb7c3f5e6..0afdc08119c8 100644
> --- a/include/linux/damon.h
> +++ b/include/linux/damon.h
> @@ -10,6 +10,7 @@
>
> #include <linux/memcontrol.h>
> #include <linux/mutex.h>
> +#include <linux/prandom.h>
> #include <linux/time64.h>
> #include <linux/types.h>
> #include <linux/random.h>
> @@ -843,8 +844,35 @@ struct damon_ctx {
>
> struct list_head adaptive_targets;
> struct list_head schemes;
> +
> + /*
> + * Per-ctx lockless PRNG state for damon_rand_fast(). Seeded from
> + * get_random_u64() in damon_new_ctx(). Owned exclusively by the
> + * kdamond thread of this ctx, so no locking is required.
> + */
> + struct rnd_state rnd_state;
> };
>
> +/*
> + * damon_rand_fast - per-ctx PRNG variant of damon_rand() for hot paths.
> + *
> + * Uses the lockless lfsr113 state kept in @ctx->rnd_state. Safe because
> + * kdamond is the single consumer of a given ctx, so no synchronization
> + * is required. Quality is sufficient for statistical sampling; do NOT
> + * use for any security-sensitive randomness.
> + *
> + * Range mapping uses Lemire's (u64)rnd * span >> 32 to avoid a division;
> + * bias is bounded by span / 2^32, negligible for DAMON.
> + */
> +static inline unsigned long damon_rand_fast(struct damon_ctx *ctx,
> + unsigned long l, unsigned long r)
> +{
> + u32 rnd = prandom_u32_state(&ctx->rnd_state);
> + u32 span = (u32)(r - l);
> +
> + return l + (unsigned long)(((u64)rnd * span) >> 32);
> +}

As Sashiko pointed out [2], we may better to return 'unsigned long' from this
function. Can this algorithm be extended for that?

> +
> static inline struct damon_region *damon_next_region(struct damon_region *r)
> {
> return container_of(r->list.next, struct damon_region, list);
> diff --git a/mm/damon/core.c b/mm/damon/core.c
> index 3dbbbfdeff71..c3779c674601 100644
> --- a/mm/damon/core.c
> +++ b/mm/damon/core.c
> @@ -607,6 +607,8 @@ struct damon_ctx *damon_new_ctx(void)
> INIT_LIST_HEAD(&ctx->adaptive_targets);
> INIT_LIST_HEAD(&ctx->schemes);
>
> + prandom_seed_state(&ctx->rnd_state, get_random_u64());
> +
> return ctx;
> }
>
> diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
> index 5cdcc5037cbc..b5e1197f2ba2 100644
> --- a/mm/damon/paddr.c
> +++ b/mm/damon/paddr.c
> @@ -48,12 +48,12 @@ static void damon_pa_mkold(phys_addr_t paddr)
> folio_put(folio);
> }
>
> -static void __damon_pa_prepare_access_check(struct damon_region *r,
> - unsigned long addr_unit)
> +static void __damon_pa_prepare_access_check(struct damon_ctx *ctx,
> + struct damon_region *r)

Let's keep 'r' on the first line, and update the second line without indent
change.

> {
> - r->sampling_addr = damon_rand(r->ar.start, r->ar.end);
> + r->sampling_addr = damon_rand_fast(ctx, r->ar.start, r->ar.end);
>
> - damon_pa_mkold(damon_pa_phys_addr(r->sampling_addr, addr_unit));
> + damon_pa_mkold(damon_pa_phys_addr(r->sampling_addr, ctx->addr_unit));
> }
>
> static void damon_pa_prepare_access_checks(struct damon_ctx *ctx)
> @@ -63,7 +63,7 @@ static void damon_pa_prepare_access_checks(struct damon_ctx *ctx)
>
> damon_for_each_target(t, ctx) {
> damon_for_each_region(r, t)
> - __damon_pa_prepare_access_check(r, ctx->addr_unit);
> + __damon_pa_prepare_access_check(ctx, r);
> }
> }
>
> diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c
> index b069dbc7e3d2..6cf06ffdf880 100644
> --- a/mm/damon/vaddr.c
> +++ b/mm/damon/vaddr.c
> @@ -332,10 +332,11 @@ static void damon_va_mkold(struct mm_struct *mm, unsigned long addr)
> * Functions for the access checking of the regions
> */
>
> -static void __damon_va_prepare_access_check(struct mm_struct *mm,
> - struct damon_region *r)
> +static void __damon_va_prepare_access_check(struct damon_ctx *ctx,
> + struct mm_struct *mm,
> + struct damon_region *r)

Let's keep the first line and the the indentation as were, and add 'ctx'
argument to the end.

> {
> - r->sampling_addr = damon_rand(r->ar.start, r->ar.end);
> + r->sampling_addr = damon_rand_fast(ctx, r->ar.start, r->ar.end);
>
> damon_va_mkold(mm, r->sampling_addr);
> }
> @@ -351,7 +352,7 @@ static void damon_va_prepare_access_checks(struct damon_ctx *ctx)
> if (!mm)
> continue;
> damon_for_each_region(r, t)
> - __damon_va_prepare_access_check(mm, r);
> + __damon_va_prepare_access_check(ctx, mm, r);
> mmput(mm);
> }
> }
> --
> 2.43.0
>
>

[1] https://lkml.kernel.org/r/20250303221726.484227-1-sj@kernel.org
[2] https://lore.kernel.org/20260423190841.821E4C2BCAF@smtp.kernel.org

Thanks,
SJ