[dpdk-dev] 回复: [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx iteration

[Feifei Wang <Feifei.Wang2@arm.com>]

> -----邮件原件-----
> 发件人: dev <dev-bounces@dpdk.org> 代表 Ananyev, Konstantin
> 发送时间: Tuesday, October 26, 2021 8:57 PM
> 收件人: Feifei Wang <Feifei.Wang2@arm.com>
> 抄送: dev@dpdk.org; nd <nd@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> 主题: Re: [dpdk-dev] [PATCH v5 4/5] lib/bpf: use wait event scheme for Rx/Tx
> iteration
> 
> 
> > Hi Feifei,
> >
> > > > Instead of polling for cbi->use to be updated, use wait event scheme.
> > > >
> > > > Furthermore, delete 'const' for 'bpf_eth_cbi_wait'. This is
> > > > because of a compilation error:
> > > > ------------------------------------------------------------------
> > > > -----
> > > > ../lib/eal/include/rte_common.h:36:13: error: read-only variable ‘value’
> > > > used as ‘asm’ output
> > > >    36 | #define asm __asm__
> > > >       |             ^~~~~~~
> > > >
> > > > ../lib/eal/arm/include/rte_pause_64.h:66:3: note: in expansion of
> macro ‘asm’
> > > >    66 |   asm volatile("ldaxr %w[tmp], [%x[addr]]" \
> > > >       |   ^~~
> > > >
> > > > ../lib/eal/arm/include/rte_pause_64.h:96:3: note: in expansion of
> > > > macro ‘__LOAD_EXC_32’
> > > >    96 |   __LOAD_EXC_32((src), dst, memorder)     \
> > > >       |   ^~~~~~~~~~~~~
> > > >
> > > > ../lib/eal/arm/include/rte_pause_64.h:167:4: note: in expansion of
> > > > macro ‘__LOAD_EXC’
> > > >   167 |    __LOAD_EXC((addr), value, memorder, size) \
> > > >       |    ^~~~~~~~~~
> > > >
> > > > ../lib/bpf/bpf_pkt.c:125:3: note: in expansion of macro ‘rte_wait_event’
> > > >   125 |   rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > > ------------------------------------------------------------------
> > > > -----
> > > >
> > > > Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> > > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > > ---
> > > >  lib/bpf/bpf_pkt.c | 11 ++++-------
> > > >  1 file changed, 4 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/lib/bpf/bpf_pkt.c b/lib/bpf/bpf_pkt.c index
> > > > 6e8248f0d6..213d44a75a 100644
> > > > --- a/lib/bpf/bpf_pkt.c
> > > > +++ b/lib/bpf/bpf_pkt.c
> > > > @@ -111,9 +111,9 @@ bpf_eth_cbi_unuse(struct bpf_eth_cbi *cbi)
> > > >   * Waits till datapath finished using given callback.
> > > >   */
> > > >  static void
> > > > -bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi)
> > > > +bpf_eth_cbi_wait(struct bpf_eth_cbi *cbi)
> > >
> > > Hi, Konstantin
> > >
> > > For this bpf patch, I delete 'const' through this is contrary to
> > > what we discussed earlier. This is because if  we keep 'constant' here and
> use 'rte_wait_event'
> > > new macro, compiler will report error. And earlier the arm version
> > > cannot be compiled due to I forgot enable "wfe" config in the meson file,
> so this issue can not happen before.
> >
> >
> > Honestly, I don't understand why we have to remove perfectly valid 'const'
> qualifier here.
> > If this macro can't be used with pointers to const (still don't
> > understand why), then let's just not use this macro here.
> > Strictly speaking I don't see much benefit here from it.
> >
> > >
> > > >  {
> > > > -	uint32_t nuse, puse;
> > > > +	uint32_t puse;
> > > >
> > > >  	/* make sure all previous loads and stores are completed */
> > > >  	rte_smp_mb();
> > > > @@ -122,11 +122,8 @@ bpf_eth_cbi_wait(const struct bpf_eth_cbi
> > > > *cbi)
> > > >
> > > >  	/* in use, busy wait till current RX/TX iteration is finished */
> > > >  	if ((puse & BPF_ETH_CBI_INUSE) != 0) {
> > > > -		do {
> > > > -			rte_pause();
> > > > -			rte_compiler_barrier();
> > > > -			nuse = cbi->use;
> > > > -		} while (nuse == puse);
> > > > +		rte_wait_event(&cbi->use, UINT32_MAX, ==, puse,
> > > > +				__ATOMIC_RELAXED);
> 
> After another thought, if we do type conversion at macro invocation time:
> 
> bpf_eth_cbi_wait(const struct bpf_eth_cbi *cbi) {
>   ...
>   rte_wait_event((uint32_t *)&cbi->use, UINT32_MAX, ==, puse,
> __ATOMIC_RELAXED);
> 
> would that help?

I try to with this and it will report compiler warning:
' cast discards ‘const’ qualifier'.
I think this is due to that in rte_wait_event macro, we use
typeof(*(addr)) value = 0;
 and value is defined as "const uint32_t",
but it should be able to be updated.

Furthermore, this reflects the limitations of the new macro, it cannot be applied
when 'addr' is type of 'const'. Finally, I think I should give up the change for "bpf".
> 
> 
> > > >  	}
> > > >  }
> > > >
> > > > --
> > > > 2.25.1