linux-kernel - Re: [patch V2 14/37] rseq: Cache CPU ID and MM CID values

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Message-ID: <130e2d99-6c8f-471e-9a72-d858431741ee@efficios.com>
Date: Mon, 25 Aug 2025 14:19:34 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 14/37] rseq: Cache CPU ID and MM CID values

On 2025-08-23 12:39, Thomas Gleixner wrote:
> In preparation for rewriting RSEQ exit to user space handling provide
> storage to cache the CPU ID and MM CID values which were written to user
> space. That prepares for a quick check, which avoids the update when
> nothing changed.

What should we do about the numa node_id field ?

On pretty much all arch except powerpc (AFAIK) it's invariant for
the topology, so derived from cpu_id.

On powerpc, we could perhaps reset the cached cpu_id to ~0U for
each thread to trigger an update ? Or just don't care about this ?

Thanks,

Mathieu

> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> ---
>   include/linux/rseq.h        |    3 +++
>   include/linux/rseq_types.h  |   19 +++++++++++++++++++
>   include/linux/sched.h       |    1 +
>   include/trace/events/rseq.h |    4 ++--
>   kernel/rseq.c               |    4 ++++
>   5 files changed, 29 insertions(+), 2 deletions(-)
> 
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -64,11 +64,13 @@ static inline void rseq_fork(struct task
>   		t->rseq = NULL;
>   		t->rseq_len = 0;
>   		t->rseq_sig = 0;
> +		t->rseq_ids.cpu_cid = ~0ULL;
>   		t->rseq_event.all = 0;
>   	} else {
>   		t->rseq = current->rseq;
>   		t->rseq_len = current->rseq_len;
>   		t->rseq_sig = current->rseq_sig;
> +		t->rseq_ids.cpu_cid = ~0ULL;
>   		t->rseq_event = current->rseq_event;
>   	}
>   }
> @@ -78,6 +80,7 @@ static inline void rseq_execve(struct ta
>   	t->rseq = NULL;
>   	t->rseq_len = 0;
>   	t->rseq_sig = 0;
> +	t->rseq_ids.cpu_cid = ~0ULL;
>   	t->rseq_event.all = 0;
>   }
>   
> --- a/include/linux/rseq_types.h
> +++ b/include/linux/rseq_types.h
> @@ -27,4 +27,23 @@ struct rseq_event {
>   	};
>   };
>   
> +/*
> + * struct rseq_ids - Cache for ids, which need to be updated
> + * @cpu_cid:	Compound of @cpu_id and @mm_cid to make the
> + *		compiler emit a single compare on 64-bit
> + * @cpu_id:	The CPU ID which was written last to user space
> + * @mm_cid:	The MM CID which was written last to user space
> + *
> + * @cpu_id and @mm_cid are updated when the data is written to user space.
> + */
> +struct rseq_ids {
> +	union {
> +		u64		cpu_cid;
> +		struct {
> +			u32	cpu_id;
> +			u32	mm_cid;
> +		};
> +	};
> +};
> +
>   #endif
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1406,6 +1406,7 @@ struct task_struct {
>   	u32				rseq_len;
>   	u32				rseq_sig;
>   	struct rseq_event		rseq_event;
> +	struct rseq_ids			rseq_ids;
>   # ifdef CONFIG_DEBUG_RSEQ
>   	/*
>   	 * This is a place holder to save a copy of the rseq fields for
> --- a/include/trace/events/rseq.h
> +++ b/include/trace/events/rseq.h
> @@ -21,9 +21,9 @@ TRACE_EVENT(rseq_update,
>   	),
>   
>   	TP_fast_assign(
> -		__entry->cpu_id = raw_smp_processor_id();
> +		__entry->cpu_id = t->rseq_ids.cpu_id;
>   		__entry->node_id = cpu_to_node(__entry->cpu_id);
> -		__entry->mm_cid = task_mm_cid(t);
> +		__entry->mm_cid = t->rseq_ids.mm_cid;
>   	),
>   
>   	TP_printk("cpu_id=%d node_id=%d mm_cid=%d", __entry->cpu_id,
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -184,6 +184,10 @@ static int rseq_update_cpu_node_id(struc
>   	rseq_unsafe_put_user(t, node_id, node_id, efault_end);
>   	rseq_unsafe_put_user(t, mm_cid, mm_cid, efault_end);
>   
> +	/* Cache the user space values */
> +	t->rseq_ids.cpu_id = cpu_id;
> +	t->rseq_ids.mm_cid = mm_cid;
> +
>   	/*
>   	 * Additional feature fields added after ORIG_RSEQ_SIZE
>   	 * need to be conditionally updated only if
> 


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com