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

Jiayuan Chen
2026-04-24T02:30:23Z

Hello SJ,

Thank you for the review.

On 4/24/26 9:36 AM, SeongJae Park wrote:
> 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.

We use DAMON paddr on a 2 TiB host for per-cgroup hot/cold page
classification.  Target cgroups can be as small as 1-2% of total
memory (tens of GiB).

With the default max_nr_regions=1000, each region covers ~2 GiB on
average, which is often larger than the entire target cgroup.
Adaptive split cannot carve out cgroup-homogeneous regions in that
regime: each region's nr_accesses averages the (small) cgroup
fraction with the surrounding non-cgroup pages, and the cgroup's
access signal gets washed out.

Raising max_nr_regions to 10k-20k gives the adaptive split enough
spatial resolution that cgroup-majority regions can form at cgroup
boundaries (allocations have enough physical locality in practice for
this to work -- THP, per-node allocation, etc.).  At that region
count, damon_rand() starts showing up at the top of kdamond profiles,
which is what motivated this patch.

> 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.

Thanks for pointing to intervals auto-tuning - we do use it.  But it

trades sampling frequency against monitoring overhead; it cannot
change the spatial resolution.  With N=1000 regions on a 2 TiB host,
a 20 GiB cgroup cannot be resolved no matter how we tune sample_us /
aggr_us, because the region boundary itself averages cgroup and

non-cgroup pages together.

So raising max_nr_regions and making the per-region overhead cheap
are complementary to interval auto-tuning, not redundant with it.

>> 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?

Sashiko is correct that (u32)(r - l) truncates spans greater than 4 GiB.

This is a pre-existing limitation of damon_rand() itself, which
passes r - l to get_random_u32_below() (a u32 parameter) and
truncates the same way.  My patch makes that truncation
explicit but does not introduce a new bug.

We can restrict the region size when config or just allow spans greater
than 4G.

static inline unsigned long damon_rand_fast(struct damon_ctx *ctx,
unsigned long l, unsigned long r)
{
unsigned long span = r - l;
u64 rnd;

if (likely(span <= U32_MAX)) {
rnd = prandom_u32_state(&ctx->rnd_state);
return l + (unsigned long)((rnd * span) >> 32);
}
rnd = ((u64)prandom_u32_state(&ctx->rnd_state) << 32) |
prandom_u32_state(&ctx->rnd_state);

// same as 'l + (unsigned long)((rnd * span) >> 32);'
return l + mul_u64_u64_shr(rnd, span, 64);
}

>> +
>> 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.
>
Thanks
>> {
>> - 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