linux-kernel - Re: [PATCH 5/6] syslets: add generic syslets infrastructure

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Date:	Fri, 7 Dec 2007 14:55:05 +0300
From:	Evgeniy Polyakov <johnpol@....mipt.ru>
To:	Zach Brown <zach.brown@...cle.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Ulrich Drepper <drepper@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@....com.au>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dan Williams <dan.j.williams@...il.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Simon Holm Thogersen <odie@...aau.dk>,
	suresh.b.siddha@...el.com
Subject: Re: [PATCH 5/6] syslets: add generic syslets infrastructure

Hi Zach.

On Thu, Dec 06, 2007 at 03:20:18PM -0800, Zach Brown (zach.brown@...cle.com) wrote:
> +/*
> + * XXX todo:
> + *  - do we need all this '*cur = current' nonsense?
> + *  - try to prevent userspace from submitting too much.. lazy user ptr read?
> + *  - explain how to deal with waiting threads with stale data in current
> + *  - how does userspace tell that a syslet completion was lost?
> + *  	provide an -errno argument to the userspace return function?
> + */
> +
> +/*
> + * These structs are stored on the kernel stack of tasks which are waiting to
> + * return to userspace.  They are linked into their parent's list of syslet
> + * children stored in 'syslet_tasks' in the parent's task_struct.
> + */
> +struct syslet_task_entry {
> +	struct task_struct *task;
> +	struct list_head item;
> +};
> +
> +/*
> + * syslet_ring doesn't have any kernel-side storage.  Userspace allocates them
> + * in their address space and initializes their fields and then passes them to
> + * the kernel.
> + *
> + * These hashes provide the kernel-side storage for the wait queues which
> + * sys_syslet_ring_wait() uses and the mutex which completion uses to serialize
> + * the (possible blocking) ordered writes of the completion and kernel head
> + * index into the ring.
> + *
> + * We chose the bucket that supports a given ring by hashing a u32 that
> + * userspace sets in the ring.
> + */
> +#define SYSLET_HASH_BITS (CONFIG_BASE_SMALL ? 4 : 8)
> +#define SYSLET_HASH_NR (1 << SYSLET_HASH_BITS)
> +#define SYSLET_HASH_MASK (SYSLET_HASH_NR - 1)
> +static wait_queue_head_t syslet_waitqs[SYSLET_HASH_NR];
> +static struct mutex syslet_muts[SYSLET_HASH_NR];

Why do you care about hashed tables scalability and not using trees?

-- 
	Evgeniy Polyakov
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