On 05/23/2012 02:46 AM, Andrew Morton wrote:
Here, we're open-coding kinda-test_bit(). Why do that? These flags are
modified with set_bit() and friends, so we should read them with the
matching test_bit()?
My reasoning was to be as cheap as possible, as you noted yourself two
paragraphs below.
Also, these bool-returning functions will return values other than 0
and 1. That probably works OK and I don't know what the C standards
and implementations do about this. But it seems unclean and slightly
risky to have a "bool" value of 32! Converting these functions to use
test_bit() fixes this - test_bit() returns only 0 or 1.
test_bit() is slightly more expensive than the above. If this is
considered to be an issue then I guess we could continue to use this
approach. But I do think a code comment is needed, explaining and
justifying the unusual decision to bypass the bitops API. Also these
functions should tell the truth and return an "int" type.
>
> +static void disarm_static_keys(struct mem_cgroup *memcg)
> +{
> + disarm_sock_keys(memcg);
> +}
Why does this function exist? Its single caller could call
disarm_sock_keys() directly.
It exists to make it clear that this is the point in which static keys
should be disabled. I already have a patchset that introduces other
static keys, that should, of course, also be disabled here.
I am totally fine with calling directly disarm_sock_keys(), and then in
that series wrap it in disarm_static_keys, IOW, defer its introduction,
if that's how you prefer.
> static void drain_all_stock_async(struct mem_cgroup *memcg);
>
> static struct mem_cgroup_per_zone *
> @@ -4836,6 +4854,13 @@ static void free_work(struct work_struct *work)
> int size = sizeof(struct mem_cgroup);
>
> memcg = container_of(work, struct mem_cgroup, work_freeing);
> + /*
> + * We need to make sure that (at least for now), the jump label
> + * destruction code runs outside of the cgroup lock.
This is a poor comment - it failed to tell the reader*why* that code
must run outside the cgroup lock.
Ok, will update.
> schedule_work()
> + * will guarantee this happens. Be careful if you need to move this
> + * disarm_static_keys around
It's a bit difficult for the reader to be careful when he isn't told
what the risks are.
Ok, will update.
> + */
> + disarm_static_keys(memcg);
> if (size< PAGE_SIZE)
> kfree(memcg);
> else
> diff --git a/net/ipv4/tcp_memcontrol.c b/net/ipv4/tcp_memcontrol.c
> index 1517037..3b8fa25 100644
> --- a/net/ipv4/tcp_memcontrol.c
> +++ b/net/ipv4/tcp_memcontrol.c
> @@ -74,9 +74,6 @@ void tcp_destroy_cgroup(struct mem_cgroup *memcg)
> percpu_counter_destroy(&tcp->tcp_sockets_allocated);
>
> val = res_counter_read_u64(&tcp->tcp_memory_allocated, RES_LIMIT);
> -
> - if (val != RESOURCE_MAX)
> - static_key_slow_dec(&memcg_socket_limit_enabled);
> }
> EXPORT_SYMBOL(tcp_destroy_cgroup);
>
> @@ -107,10 +104,33 @@ static int tcp_update_limit(struct mem_cgroup *memcg, u64 val)
> tcp->tcp_prot_mem[i] = min_t(long, val>> PAGE_SHIFT,
> net->ipv4.sysctl_tcp_mem[i]);
>
> - if (val == RESOURCE_MAX&& old_lim != RESOURCE_MAX)
> - static_key_slow_dec(&memcg_socket_limit_enabled);
> - else if (old_lim == RESOURCE_MAX&& val != RESOURCE_MAX)
> - static_key_slow_inc(&memcg_socket_limit_enabled);
> + if (val == RESOURCE_MAX)
> + clear_bit(MEMCG_SOCK_ACTIVE,&cg_proto->flags);
> + else if (val != RESOURCE_MAX) {
> + /*
> + * The active bit needs to be written after the static_key update.
> + * This is what guarantees that the socket activation function
> + * is the last one to run. See sock_update_memcg() for details,
> + * and note that we don't mark any socket as belonging to this
> + * memcg until that flag is up.
> + *
> + * We need to do this, because static_keys will span multiple
> + * sites, but we can't control their order. If we mark a socket
> + * as accounted, but the accounting functions are not patched in
> + * yet, we'll lose accounting.
> + *
> + * We never race with the readers in sock_update_memcg(), because
> + * when this value change, the code to process it is not patched in
> + * yet.
> + *
> + * The activated bit is used to guarantee that no two writers will
> + * do the update in the same memcg. Without that, we can't properly
> + * shutdown the static key.
> + */
This comment needlessly overflows 80 cols and has a pointless and
unconventional double-space indenting. I already provided a patch
which fixes this and a few other things, but that was ignored when you
did the v6.
Sorry, I missed it.
> + if (!test_and_set_bit(MEMCG_SOCK_ACTIVATED,&cg_proto->flags))
> + static_key_slow_inc(&memcg_socket_limit_enabled);
> + set_bit(MEMCG_SOCK_ACTIVE,&cg_proto->flags);
> + }
So here are suggested changes from*some* of the above discussion.
Please consider, incorporate, retest and send us a v7?
How do you want me to do it? Should I add your patch ontop of mine,
and then another one that tweaks whatever else is left, or should I just
merge those changes into the patches I have?
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